Producing a diamond semiconductor by implanting dopant using ion implantation

ABSTRACT

A process of producing a diamond thin-film includes implanting dopant into a diamond by an ion implantation technique, forming a protective layer on at least part of the surface of the ion-implanted diamond, and firing the protected ion-implanted diamond at a firing pressure of no less than 3.5 GPa and a firing temperature of no less than 600° C. A process of producing a diamond semiconductor includes implanting dopant into each of two diamonds by an ion implantation technique and superimposing the two ion-implanted diamonds on each other such that at least part of the surfaces of each of the ion-implanted diamonds makes contact with each other, and firing the ion implanted diamonds at a firing pressure of no less than 3.5 GPa and a firing temperature of no less than 600° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/023,660, filed Jan. 31, 2008, which is a divisional of U.S. patentapplication Ser. No. 11/577,678, filed Apr. 20, 2007, which is anationalization of PCT Application No. PCT/JP2006/312334, filed Jun. 20,2006, which claims priority to Japanese Patent Application Nos.2005-179751 filed Jun. 20, 2005, 2005-270541 filed Sep. 16, 2005,2005-307231 filed Oct. 21, 2005, and 2006-061838 filed Mar. 7, 2006,which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a diamond semiconductor element, andmore particularly, to a semiconductor element structure in whichoccurrence of a crystal defect peculiar to a diamond semiconductorelement is suppressed, and a process for producing the same.

BACKGROUND ART

A diamond has the highest thermal conductivity among a variety ofmaterials, and has the highest breakdown electric field strength amongsemiconductors. Accordingly, the diamond is the semiconductor materialmost suitable for a high-power semiconductor requiring high voltage,large current operation. Since holes and electrons in the diamond have ahigh mobility and saturated velocity, the diamond is suitable for ahigh-frequency semiconductor element operable in high frequencies. Ahigh-frequency diamond semiconductor element is a semiconductor elementcontrolling high power in a high frequency band including a micro-waveband region and a millimeter-wave band region.

FIGS. 5A to 5C are diagrams illustrating conventional process-steps inproducing a diamond semiconductor element. A description will be givenbelow of the process-steps in producing a diamond semiconductortransistor by the use of a conventional technique disclosed innon-patent document 1 (see non-patent document 2 regarding surfaceorientation).

First, a single-crystal diamond substrate 1-31 is prepared as shown inFIG. 5A. The surface orientation of the surface of the single-crystaldiamond substrate 1-31, which is{right arrow over (s)}  [Expression 1]has its surface orientation preciously in the [001] direction.

Then, as illustrated in FIG. 5B, crystal growth is performed on thesingle-crystal diamond substrate 1-31 to form a single-crystal diamondthin-film 1-32 thereon. In the crystal growth process-step, atwo-dimensional-form hole channel 1-33 is formed in such a manner as tobe parallel to the surface of the single-crystal diamond thin film 1-32.The surface orientation of the surface of the single-crystal diamondthin-film 1-32, which is{right arrow over (d)}  [Expression 2]and, the surface orientation of the forming face of the hole channel1-33, which is{right arrow over (c)}  [Expression 3]becomes equal to the surface orientation of the surface of thesingle-crystal diamond substrate 1-31, and thus has the surfaceorientation preciously in the [001] direction.

Then, as illustrated in FIG. 5C, a source electrode 1-34, a gateelectrode 1-35, and a drain electrode 1-36 are all formed on thesingle-crystal diamond thin-film 1-32. The longitudinal direction of thegate electrode 1-35, which is{right arrow over (g)}  [Expression 4]is the [100] direction.

The characteristics of the transistor produced by the aforementionedconventional method are disclosed in detail in non-patent document 1.The transistor characteristics data on all items with reference to thecharacteristics of a transistor having a gate length of 0.2 μm(normalized by the gate width) is disclosed. According to non-patentdocument 1, the maximum transconductance gmmax of the transistor is 150mS/mm at the most.

However, a diamond single-crystal has the problem of an extremely highdensity of a crystal defect as compared with that of anothersemiconductor, for example, silicone, gallium arsenide, indiumphosphide, gallium nitride, and the like. For this reason, the naturalphysical properties of a diamond, such as high thermal conductivity, ahigh breakdown electrical field, satisfactory high-frequencycharacteristics and the like, cannot be reflected in the transistorcharacteristics. A transistor using a diamond semiconductor is not yetin actual use. This problem is described in non-patent document 1. Inconsequence, for the achievement of the practically useful transistorusing a diamond single-crystal, what is required is to produce anelement by a method that minimizes the occurrence of the crystal defectpeculiar to a diamond.

It is a first object of the present invention to significantly suppressoccurrence of a crystal defect specific to a diamond by shifting thesurface orientation of a diamond substrate slightly from the [001]direction.

It is theoretically clear that because a diamond has the highest thermalconductivity among materials and has the highest breakdown electricfield strength among semiconductors, the diamond is the semiconductormaterial most suitable for a high-power semiconductor operable at highvoltage and large current. In addition, it is known that because adiamond has a high mobility and saturated velocity of electrons andholes, the diamond is suitable for a high-frequency semiconductorelement operable at high frequency.

FIGS. 13A to 13G illustrate process-steps in producing a conventionaldiamond semiconductor element. Gold (Au) is evaporated onto a diamondsingle-crystal thin-film 2-11 (FIG. 13A) having a two-dimensional holechannel close to the surface to form an Au thin-film 2-12 (FIG. 13B).The Au thin-film 2-12 is coated with a resist 2-13 (FIG. 13C). Then,photolithography or an electron beam is applied for exposure anddevelopment to remove part of the resist 2-13 to form an aperture in theresist 2-13 above the area in which a gate electrode will be formed(FIG. 13D). Then, a sample is immersed in an Au etchant to etch aportion of the Au thin-film 2-12 close to the aperture in the resist2-13 (FIG. 13 E).

As illustrated in FIG. 13E, the etching is performed on the portion ofthe surface of the Au thin-film 2-12 exposed by the aperture of theresist 2-13, then proceeds from there in the depth direction (thedirection at right angles to the diamond single-crystal thin-film 2-11)and simultaneously in the lateral direction (the direction horizontal tothe diamond single-crystal thin-film 2-11). For this reason, an area ofthe Au thin-film 2-12 below the resist 2-13 is also cut away. Theportion cut away or, as it were, hollowed out is called the “undercut”.When the Au thin-film 2-12 is etched in the lateral direction by theetchant in this manner, because the bonding strength between the Authin-film 2-12 and the resist 2-13 is greater than the bonding strengthbetween the Au thin-film 2-12 and the diamond single-crystal thin-film2-11, the etching velocity in the lateral diction is slower close to theresist 2-13 and faster close to the diamond single-crystal thin-film2-11. For this reason, an angle θ of the end face of the etched portionof the Au thin-film 2-12 is about 45 degrees. In other words, eachsection of the Au thin-film 2-12 divided into two by the etching has aninverted mesa shape having the upper side wider than the lower side.

Next, Al (aluminum) is evaporated (FIG. 13F). The Al, which passesthrough the apertures of the resist 2-13 and the Au thin-film 2-12 to beevaporated directly onto the surface of the diamond single-crystalthin-film 2-11, and the Al, which is evaporated onto the resist 2-13,respectively form Al thin-films 2-15G, 2-15. Then, a sample is immersedin a liftoff fluid for the liftoff of the resist 2-13 to remove theresist 2-13 and the Al thin-film 2-15 evaporated thereon (FIG. 13G). Atthis stage, one of the Au thin-films 2-12 is defined as a sourceelectrode 2-16S and the other thin-film 2-12 is defined as a drainelectrode 2-16D, and also, the Al thin-film 2-15G remaining on thediamond single-crystal thin-film 2-11 is defined as a gate electrode2-17G. At this point, the thickness t_(S), t_(D) is 0.6 μm, and the gatelength d_(G) corresponding to the length from the end of the gateelectrode 2-17G close to the source to the end close to the drain is 0.2μm.

The diamond semiconductor has the physically derived limitation that thechannels in which the electron and the hole of a transistor travel arerequired to be located within 0.1 μm from the surface unlike othersemiconductors, for example, silicone, gallium arsenide, indium, galliumnitride, and the like (see non-patent document 3).

Under this theoretically limitation, a requirement for an increase inthe transconductance g_(m), which is the degree of amplification of thediamond semiconductor element, to produce an enhancement up to apractical level in the maximum oscillation frequency f_(max), which isthe upper limit to operation frequency in the high-frequencycharacteristics, is to reduce the source-gate electrode distance d_(SG)between an end of the face of the source electrode 2-16S making contactwith the diamond single-crystal thin-film 2-11 and the end of the gateelectrode 2-17G close to the source, and the gate-drain electrodedistance d_(GD) between the end of the gate electrode 2-17G close to thedrain and an end of the face of the drain electrode 2-16D making contactwith the diamond single-crystal thin-film 2-11. This presents fewerproblems in the cases of other semiconductors, but in the diamondsemiconductor element this is a critical problem resulting from itsphysical properties that needs to be solved. In addition, a reduction asgreat as possible in the gate length d_(G) in FIG. 13G is also requiredfor an increase in the maximum oscillation frequency f_(max).

Non-patent document 4 discloses that the source-gate electrode distanced_(SG) and the gate-drain electrode distance d_(GD) are respectivelyfrom 1.3 μm to 1.4 μm, because the distance between the source electrode2-16S and the drain electrode 2-16D is 2.6 μm or 2.7 μm, and the gatelength d_(G) is 0.2 μm.

At the same time, for the prevention of an unwanted voltage drop fromoccurring in transistor operation, the source electrode resistance andthe drain electrode resistance are required to be reduced as much aspossible. For a reduction in the source electrode resistance and thedrain electrode resistance, the thickness t_(S) of the source electrode2-16S and the thickness t_(D) of the drain electrode 2-16D are requiredto be increased as much as possible.

However, in the process of etching the Au thin-film 2-12, the invertedmesa structure having an angle θ of about 45 degrees, as shown in FIG.13E, occurs at the end faces of the source electrode 2-16S and the drainelectrode 2-16D close to the gate electrode 17G. This gives rise to theproblem of the impossibility of reducing the source-gate electrodedistance d_(SG) and the gate-drain electrode distance d_(GD) so as to besmaller than the thickness t_(S) of the source electrode 2-16S and thethickness t_(D) of the drain electrode 2-16D. In other words, theconventional art is incapable of simultaneously satisfying the tworequirements, “a reduction in the source-gate electrode distance and thegate-drain electrode distance” and “an increase in the thickness of thesource electrode and the thickness of the drain electrode”.

FIGS. 14A to 14C show the characteristics of a diamond transistorproduced by a conventional process. They are the results disclosed innon-patent document 3, all of which are standardized with reference tothe characteristics of a transistor having a gate length of 0.2 μm. Inthe drain current-voltage characteristics shown in FIG. 14A, the maximumdrain current normalized by the gate length d_(G) is 0.35 A/mm at themost. In the dependence of the transconductance g_(m) on the gatevoltage V_(G) (transfer characteristics) shown in FIG. 14B, the maximumtransconductance g_(mmax) normalized by the gage length d_(G) is 150mS/mm at the most. Further, in the dependence of the power gain U on thefrequency f shown in FIG. 14C, the maximum oscillation frequency f_(max)which is an upper limit to the operation frequency is 81 GHz at themost. The drain breakdown voltage, which is not shown in figures, is 45Vat the most.

Accordingly, it is a second object of the present invention to achievethe compatibility between “a reduction in the source-gate electrodedistance d_(SG) and the gate-drain electrode distance d_(GD)” and “anincrease in the thickness t_(S) of the source electrode and thethickness t_(D) of the drain electrode” to increase the maximumoscillation frequency f_(max) for a great improvement in thecharacteristics of a diamond field-effect transistor, and also to bringthe voltage drop down, thus reaching a practical level.

Next, a process for producing a conventional diamond single-crystalthin-film will be described using FIGS. 22A and 22B. A diamondsingle-crystal substrate 3-1 having surface orientation (100) isprepared (FIG. 22A). Then, a microwave plasma CVD (chemical vapordeposition) apparatus is used to deposit a diamond single-crystalthin-film 3-2 of about 1 μm to 5 μm thickness onto the diamondsingle-crystal substrate 3-1 at a substrate temperature of 700° C. usingmethane as a reaction gas (FIG. 22B). The surface of the diamondthin-film obtained by the CVD technique is hydrogen-terminated in anas-grown state and has surface conduction properties, and functions as aP-type semiconductor.

Non-patent document 5 describes that, in order to improve thecrystallinity of a diamond thin-film deposited on a silicone substrate,the above diamond thin-film is placed in a ceramic tube in which avacuum of 1×10⁻⁶ Torr is produced, and high-temperature annealing of1000° C. or more is performed in the vacuum.

Then, the diamond single-crystal thin-film produced by the process forproducing the conventional diamond single-crystal thin-film described inFIGS. 22A and 22B has an average mobility of about 800 cm²/Vs at roomtemperature, and a high quality thin-film is obtained with satisfactoryreproducibility. However, impurities and a large number of crystaldefects such as a growth hillock, abnormal growth particles and the likeexist in the above diamond single-crystal thin-film.

In non-patent document 5, when the temperature at which the annealing isperformed reaches 1200° C. or more, band-A emissions associated withdeterioration of the diamond thin-film (emission resulting from defects)are increased. In short, in patent document 2, deterioration incrystallinity is increased at 1200° C. or more.

Because the higher the annealing temperature becomes, the more thecrystallinity is improved, the annealing is desirably performed athigher temperatures. However, in non-patent document 5, if thetemperature is increased for an improvement in crystallinity, when thetemperature reaches a certain degree (1200° C.) or more, the conversionto graphite progresses in the diamond thin-film, resulting in anincrease in deterioration in crystallinity.

Accordingly, it is a third object of the present invention to provide adiamond-thin-film producing process which is capable of reducing crystaldefects, impurities and the like existing in a diamond thin-film toproduce a high quality diamond thin-film.

It is known that a diamond has physical characteristics as asemiconductor superior to those of silicon (Si). It is recognized in atheory that the diamond element has characteristics regardinghigh-temperature operation five times that of the Si element,high-voltage performance 30 times that of the Si element and an increasein speed three times that of the Si element. For this reason, it isexpected that the diamond will realize a high-output device having ahigh thermal conductivity and a breakdown electric field strength, ahigh-frequency device having a high carrier mobility and a highsaturated drift velocity, and the like. In other words, because afield-effect transistor (FET) or a bipolar transistor uses a diamondsemiconductor, an electron element capable of driving at a highfrequency for high-power operation, significantly exceeding theconventional semiconductors, is provide. In addition, it is clear intheory that the realization of a semiconductor laser and a lightemitting diode using a diamond semiconductor realizes a high intensitylight emitting element with a wavelength of 225 nm in the ultravioletregion (see non-patent document 6).

A diamond has a band gap of 5.5 eV and is originally an insulator, but,as in the case of Si, if the diamond is doped with B which is a IIIgroup element, an acceptor level occurs, resulting logically in a p-typesemiconductor.

Since a p-type semiconductor layer in a transistor or optical devicestructure has a high resistance, as in the case of an insulator when thehole concentration is less than 1.0×10¹⁵ cm⁻³, the p-type semiconductorlayer does not fully function as a p-type semiconductor, and is thus ofno use. Also, since a p-type semiconductor layer in a transistor oroptical device structure shows a metallic electrical conduction when thedensity of the dopant element exceeds 1.0×10²¹ cm⁻³, in this case, thep-type semiconductor layer also does not fully functions as a p-typesemiconductor, and is thus of no use. Thus, in the p-type semiconductorlayer the hole concentration is required to be 1.0×10¹⁵ cm⁻³ or more andthe dopant element concentration to be 1.0×10²¹ cm⁻³ or less.

Also, because the hole concentration and the dopant elementconcentration in a semiconductor depend on temperatures, it is importantto satisfy the above requirements in operational temperatures aroundroom temperature (300K) in order to ensure the practicality of thedevice. In addition, in high power use such as in appliances, electricalapparatuses, industrial equipment and the like, operation in ahigh-temperature condition is particularly required. For this reason,there is a necessity to satisfy the above requirements in, for example,about 500K which is higher than room temperature.

However, as shown in FIG. 38, the conventional technique of doping adiamond with boron (B) has the problem of only a hole concentration of6×10¹⁴ cm⁻³ being obtained at room temperature (300 k) even when theB-atom concentration is 1.0×10²¹ cm⁻³. In FIG. 38, the horizontal axisrepresents the measurement temperature (K) and the vertical axisrepresents the hole concentration (cm⁻³) in a conventional p-typediamond semiconductor, in which the measured values at each B-atomconcentration in the conventional p-type diamond semiconductor areplotted. These values do not satisfy the 1.0×10¹⁵ cm⁻³ required forpractical use at 300K. As a result, there arises the problem of theincapability of the practical use of a diamond semiconductor as atransistor or an optical device.

As means for increasing the hole concentration at about 300K of thediamond semiconductor, a greater increase in the B-atom concentrationthan 1.0×10²¹ cm⁻³ is conceivable. However, the crystal quality of adiamond becomes poor as the B-atom concentration becomes greater than1.0×10²¹ cm⁻³, so that the diamond loses its semiconductor properties,resulting in the problem that it is of no practical use.

Accordingly, it is a fourth object of the present invention to provide apractically useful p-type diamond semiconductor having a holeconcentration of 1.0×10¹⁵ cm⁻³ or more at room temperature (300K) ormore and a dopant atom concentration of 1.0×10²¹ cm⁻³ or less, and toprovide a process for producing the same.

A diamond is a semiconductor having both the highest thermalconductivity (22 W/cmK) and the highest breakdown electric field (>10MV/cm) among substances as described above, and also a high carriermobility (electron: 4500 cm²/Vs, hole: 3800 cm²/Vs), and if highlyefficient doping is accomplished, a transistor operating at a highfrequency and high output surpassing that of Si, GaAs, GaN is realized.

One of the methods of doping a diamond is an ion implantation technique.The ion implantation technique is a method for accelerating impuritiesat high voltage so as to lend them energy of some kV to some MV for theintroduction of impurity ions into the crystal, in which because a highenergy process is involved, damage (crystal defects, amorphous layer andthe like) occurs in the crystal in proportion to the accelerationvoltage. The damage can be removed by performing appropriatehigh-temperature annealing treatment, with the result that the dopant iselectrically activated and the semiconductor characteristics caused bythe implanted impurities emerge. However, since a diamond isthermodynamically a meta-stable layer under ordinary pressure (1atmospheric pressure), a high-quality diamond semiconductor is notobtained by the generally employed annealing process performed underordinary pressure or vacuum. For this reason, in patent document 1high-temperature annealing under high pressure is performed as describedbelow.

FIGS. 43A to 43E illustrate the process-steps in producing a diamondsemiconductor using an ion implantation technique according toconventional art. An ion implantation apparatus is used to implantdopant (boron) into a diamond single-crystal (FIG. 43A) under theconditions of an acceleration voltage of 150 kV and a dose of 1×10¹⁶cm⁻² (FIGS. 43B to 43C) and then firing (annealing) is performed for onehour at a pressure and a temperature of 5 GPa and 1700K (FIG. 43D).

However, the diamond thin-film subjected to the annealing treatment inthis manner has a high resistance, thus giving rise to the problem ofexhibiting no semiconductor characteristics. This is because etchingoccurs on the diamond surface during the process of the high-temperatureand high-pressure annealing treatment so as to cut away the layer intowhich the ion is implanted (FIG. 43D). In this manner, the conventionaltechnique has the problem of the impossibility of providing a diamondsemiconductor because the diamond layer formed by ion implantation isetched during the high-temperature and high-pressure annealing.

Accordingly, it is a fifth object of the present invention to provide aprocess for producing a diamond semiconductor which is capable ofpreventing the diamond surface from being etched by the high-temperatureand high-pressure annealing performed on ion-implanted diamond toprovide high-quality P-type and N-type diamond semiconductors whichcannot be provided by conventional processes.

-   Patent document 1: Japanese Patent Publication No. 8-15162-   Non-patent document 1: Makoto KASU et al., “High-frequency    characteristics of diamond MESFET”, Journal of the Japan society of    applied physics “Applied Physics”, vol. 73, No. 3 (March, 2004), pp.    363-367-   Non-patent document 2: C. Kittel, “Introduction to Solid State    Physics”, published by Maruzen, the fifth edition, first volume, pp.    11-22-   Non-patent document 3: Makoto KASU and 6 others, “High-frequency    characteristics of diamond MESFET”, Journal of Applied Physics, the    Japan society of applied physics, 2004, vol. 73, No. 3, pp.    0363-0367-   Non-patent document 4: M. Kasu, “Influence of epitaxy on    hydrogen-passivated diamond” Diamond and Related Materials, 2004,    No. 13, pp. 226-232-   Non-patent document 5: J. Ruan et al., “Cathodoluminescence and    annealing study of plasma—deposited polycrystalline diamond    films” J. Appl. Phys. 69(9), 1 May 1991-   Non-patent document 6: Kasu et al., “High-frequency characteristics    of diamond MESFET”, Journal of Applied Physics, 2004, vol. 73, No.    3, pp. 363-367-   Non-patent document 7: F. P. Bundy, H. P. Bovenkerk, H. M. Strong,    and R. H. Wentorf, “Diamond-Graphite Equilibrium Line from Growth    and Graphitization of Diamond”, The Journal of Chemical Physics,    August, 1961, Vol. 35m Number 2, pp. 383

SUMMARY OF THE INVENTION

In the present invention, in order to attain the above-described firstobject, an aspect of the present invention is characterized in that, ina field-effect transistor comprising a single-crystal diamond thin-film,a hole or electron channel which is formed inside the single-crystaldiamond thin-film, and a drain electrode, a gate electrode and a sourceelectrode which are formed on the single-crystal diamond thin-film, a[001] direction of the crystal axis of the single-crystal diamondthin-film is inclined with respect to surface orientation of a surfaceof the single-crystal diamond thin-film, or alternatively, a [001]direction of the crystal axis of the single-crystal diamond thin-film isinclined with respect to surface orientation of a forming face of thechannel.

The angle αd formed between the surface orientation of the surface ofthe single-crystal diamond thin-film and the [001] direction of thecrystal axis of the single-crystal diamond thin-film, or the angle αcformed between the surface orientation of the forming face of thechannel and the crystal axis [001] direction of the single-crystaldiamond thin-film may be in the range of from 0.05 degrees to 1.1degrees.

Another aspect of the present invention is characterized in that, in afield-effect transistor comprising a single-crystal diamond thin-filmformed on a substrate, a hole or electron channel which is formed insidethe single-crystal diamond thin-film, and a drain electrode, a gateelectrode and a source electrode which are formed on the single-crystaldiamond thin-film, a [001] direction of the crystal axis of thesingle-crystal diamond thin-film is inclined with respect to surfaceorientation of a surface of the single-crystal diamond thin-film, oralternatively, a [001] direction of the crystal axis of thesingle-crystal diamond thin-film is inclined with respect to surfaceorientation of a forming face of the channel.

The angle αd formed between the surface orientation of the surface ofthe single-crystal diamond thin-film and the [001] direction of thecrystal axis of the single-crystal diamond thin-film, or the angle αcformed between the surface orientation of the forming face of thechannel and the [001] direction of the crystal axis of thesingle-crystal diamond thin-film may be in the range of from 0.05degrees to 1.1 degrees.

Another aspect of the present invention is characterized in that, in afield-effect transistor comprising a single-crystal diamond thin-filmformed on a substrate, a hole or electron channel which is formed on thesingle-crystal diamond thin-film, and a drain electrode, a gateelectrode and a source electrode which are formed on the channel, a[001] direction of the crystal axis of the single-crystal diamondthin-film is inclined with respect to surface orientation of a surfaceof the channel.

The angle αc formed between the surface orientation of the surface ofthe channel and the [001] direction of the crystal axis of thesingle-crystal diamond thin-film may be in the range of from 0.05degrees to 1.1 degrees.

These substrates are a single-crystal diamond substrate, and the angleαs formed between the surface orientation of an interface between thesingle-crystal diamond substrate and the single-crystal diamondthin-film and the [001] direction of the crystal axis of thesingle-crystal diamond thin-film may be in the range of from 0.05degrees to 1.1 degrees.

The angle β formed between the longitudinal direction of these gateelectrodes and the [110] direction of the crystal axis of thesingle-crystal diamond thin-film may be in the range of from minus 30degrees to plus 30 degrees.

Another aspect of the present invention is characterized by comprising,in a process for producing a field-effect transistor comprising asingle-crystal diamond thin-film formed on a substrate, a hole orelectron channel which is formed inside the single-crystal diamondthin-film, and a drain electrode, a gate electrode and a sourceelectrode which are formed on the single-crystal diamond thin-film, thestep of forming the single-crystal diamond thin-film on the substrate;the step of forming the channel on the single-crystal diamond thin-film;the step of further forming the single-crystal diamond thin-film on thechannel thus formed; and the step of performing a grinding process on asurface of the single-crystal diamond thin-film in such a manner as toincline surface orientation of the surface of the single-crystal diamondthin-film with respect to the [001] direction of the crystal axis of thesingle-crystal diamond thin-film.

Still another aspect of the present invention is characterized bycomprising, in a process for producing a field-effect transistorcomprising a single-crystal diamond thin-film formed on a substrate, ahole or electron channel which is formed inside the single-crystaldiamond thin-film, and a drain electrode, a gate electrode and a sourceelectrode which are formed on the single-crystal diamond thin-film, thestep of forming the single-crystal diamond thin-film on the substrate;the step of performing a grinding process on a surface of thesingle-crystal diamond thin-film thus formed in such a manner as toincline the surface of the single-crystal diamond thin-film with respectto the [001] direction of a crystal axis of the single-crystal diamondthin-film; and the step of forming the channel on the single-crystaldiamond thin-film after subjection to the grinding process.

Another aspect of the present invention is characterized by comprising,in a process for producing a field-effect transistor comprising asingle-crystal diamond thin-film formed on a single-crystal diamondsubstrate, a hole or electron channel which is formed inside thesingle-crystal diamond thin-film, and a drain electrode, a gateelectrode and a source electrode which are formed on the single-crystaldiamond thin-film, the step of performing a grinding process on asurface of the single-crystal diamond substrate in such a manner as toincline surface orientation of the single-crystal diamond substrate withrespect to the [001] direction of the crystal axis of the single-crystaldiamond substrate; the step of forming the single-crystal diamondthin-film on the single-crystal diamond substrate after subjection tothe grinding process; and the step of forming the channel on thesingle-crystal diamond thin-film.

The surface orientation of conventional single-crystal diamondsubstrates points precisely in the [001] direction. However, the 001surface orientation is a surface orientation in which defects occurextremely readily. To avoid this, the surface of the diamond substrateis shifted slightly from the [001] direction, thereby making it possibleto significantly reduce the occurrence of crystal defects peculiar to adiamond. A high transconductance of the field-effect transistor isprovided.

In the present invention, in order to attain the aforementioned secondobject, an aspect of the present invention is a diamond semiconductorelement which is characterized in that, in a diamond semiconductorelement having a gate electrode, a source electrode and a drainelectrode formed on a diamond single-crystal thin-film at a distancefrom each other, the source electrode includes a first area close to thediamond single-crystal thin-film and a second area except the firstarea, where a first distance between a first end face of the first areaclose to the gate electrode and the gate electrode is no more than asecond distance between a second end face of the second area close tothe gate electrode and the gate electrode, and the drain electrodeincludes a third area close to the diamond single-crystal thin-film anda fourth area except the third area, where a third distance between athird end face of the third area close to the gate electrode and thegate electrode is no more than a fourth distance between a fourth endface of the fourth area close to the gate electrode and the gateelectrode.

The first distance may be in the range of from 0.1 μm to 10 μm, and thesecond distance may be in the range of from the first distance to 30 μm.

The third distance may be in the range of from 0.1 μm to 50 μm, and thefourth distance may be in the range of from the third distance to 50 μm.

The thickness of the first area may be in the range of from 0.01 μm to0.2 μm, and the thickness of the second area may be no less than 0.2 μm.

The thickness of the third area may be in the range of from 0.01 μm to0.2 μm, and the thickness of the fourth area may be no less than 0.2 μm.

An aspect of the present invention is a diamond semiconductor elementwhich is characterized in that, in a diamond semiconductor having a gateelectrode, a source electrode and a drain electrode formed on a diamondsingle-crystal thin-film at a distance from each other, the sourceelectrode includes, at least, a first lower metal film formed on thediamond single-crystal thin-film and a first upper metal film formed onthe first lower metal film, where a first distance between a first endface of the first lower metal film close to the gate electrode and thegate electrode is no more than a second distance between a second endface of the first upper metal film close to the gate electrode and thegate electrode, and the drain electrode includes at least a second lowermetal film formed on the diamond single-crystal thin-film and a secondupper metal film formed on the second lower metal film, where a thirddistance between a third end face of the second lower metal film closeto the gate electrode and the gate electrode is no more than a fourthdistance between a fourth end face of the second upper metal film closeto the gate electrode and the gate electrode.

The first distance may be in the range of from 0.1 μm to 10 μm, and thesecond distance may be in the range of from the first distance to 30 μm.

The third distance may be in the range of from 0.1 μm to 50 μm, and thefourth distance may be in the range of from the third distance to 50 μm.

The thickness of the first lower metal film may be in the range of from0.01 μm to 0.2 μm, and the thickness of the first upper metal film maybe no less than 0.2 μm.

The invention described in claim 10 of original PCT Application No.PCT/JP2006/312334, from which the present application claims priority,is the diamond semiconductor element described in claims 6 to 9 oforiginal PCT Application No. PCT/JP2006/312334, which is characterizedin that the thickness of the second lower metal film may be in the rangeof from 0.01 μm to 0.2 μm, and the thickness of the second upper metalfilm may be no less than 0.2 μm.

The first lower metal film may be formed of either gold or an alloyincluding gold, and the first upper metal film may be formed either ofany of the metals, gold, platinum, palladium, titanium, molybdenum andtungsten, or of an alloy including the metal.

The second lower metal film may be formed of either gold or alloyincluding gold, and the second upper metal film may be formed either ofany of the metals, gold, platinum, palladium, titanium, molybdenum andtungsten, or of an alloy including the metal.

Another aspect of the present invention is a process for producing adiamond semiconductor element, which is characterized by comprising: thestep of forming a first metal film on a diamond single-crystalthin-film; the step of forming a second metal film on the first metalfilm; the step of forming, in a first area of the second metal film, afirst aperture reaching the surface of the first metal film; the step ofetching away a part of the surface of the first metal film exposed bythe aperture to form a second aperture reaching the surface of thediamond single-crystal thin-film; and the step of forming a third metalfilm on the diamond single-crystal thin-film exposed by the secondaperture.

In the step of forming the first aperture, the first aperture may beformed such that the width of the first aperture is larger than thethickness of the second metal film.

According to the present invention, in a diamond semiconductor, thesource electrode and the drain electrode are formed in such a manner asto divide each electrode into a layer for liftoff and a layer foretching, thereby making it possible to achieve the compatibility between“a reduction in the source-gate electrode distance and the gate-drainelectrode distance” and “an increase in the thickness of the sourceelectrode and the thickness of the drain electrode” to greatly improvethe characteristics of a diamond field-effect transistor to reach apractical level.

In the present invention, in order to attain the aforementioned thirdobject, an aspect of the present invention is a process for producing adiamond thin-film, which is characterized by comprising: a firstprocess-step of forming a diamond crystal thin-film on a substrate; anda second process-step of firing the diamond crystal thin-film thusformed under a high pressure under which a diamond is stable.

Another aspect of the present invention is a process for producing adiamond thin-film, which is characterized by comprising: a firstprocess-step of forming a diamond crystal thin-film on a substrate; asecond process-step of putting two of the diamond crystal thin-film thusformed in readiness; and a third process-step of firing the two diamondcrystal thin-films, superimposed one on another such that surfaces ofthe two diamond crystal thin-films make at least partial contact witheach other, under a high pressure under which a diamond is stable.

Another aspect of the present invention is a process for producing adiamond thin-film, which is characterized by comprising: a firstprocess-step of forming a diamond crystal thin-film on a substrate; asecond process-step of either forming a protective member on at least apart of a surface of the diamond crystal thin-film thus formed, oroverlaying the at-least part with a protective member, to use theprotective member to protect the at-least part; and a third process-stepof firing the diamond crystal thin-film thus protected by the protectivemember under a high pressure under which a diamond is stable.

The protective member may be any one of the three, silicon oxide,silicon nitride or aluminum oxide.

The protective member may be titanium, tungsten, platinum, palladium ormolybdenum, or alternatively an alloy including at least one of them.

Further, the substrate may be a diamond single-crystal substrate.

The diamond crystal thin-film may be a diamond single-crystal thin-film.

Surface orientation of the diamond single-crystal substrate and thediamond single-crystal thin-film may be (111).

The diamond crystal thin-film may be a diamond polycrystal thin-film.

In the first process-step, at least either control of temperature forheating the substrate when the diamond crystal thin-film is formed, orcontrol of the ratio of the flow rate of methane to the flow rate ofhydrogen in the methane and the hydrogen used when the diamond crystalthin-film is formed may be performed.

The temperature may be controlled so as to be no less than growingtemperatures when the diamond crystal thin-film is formed, and no morethan 700° C.

The ratio may be controlled so as to exceed 0% and to be no more than0.5%.

The smoothness of the surface of the diamond crystal thin-film may be nomore than 30 nm in a region of 1 μm² in terms of average squareroughness.

The relation between a pressure P(GPa) when the firing is carried outand a temperature T(K) when the firing is carried out may satisfy therelation of the expression P≧0.71+0.0027T, and also the firing may becarried out under a pressure of P≧1.5 GPa.

In the first process-step, the diamond crystal thin-film may be formedby a microwave plasma CVD technique.

According to the present invention, the diamond crystal thin-film issubjected to high-pressure and high-temperature annealing treatment insuch a manner as to make a diamond stable, thus making it possible toreduce the crystal defects and impurities in the diamond crystalthin-film, resulting in an increase in quality of the diamond crystalthin-film.

In the present invention, in order to attain the aforementioned fourthobject, an aspect of the present invention is a p-type diamondsemiconductor, which is characterized by comprising, as a dopantelement, any of the elements, Al, Be, Ca, Cd, Ga, In, Li, Mg and Zn.

The concentration of the dopant element may be a concentration enablinga hole concentration in the p-type diamond semiconductor to be no lessthan 1.0×10¹⁵ cm⁻³ and also to be no more than 1.0×10²¹ cm⁻³ at eachtemperature.

Al may be contained as the dopant element in a concentration of no lessthan 2.6×10¹⁶ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Be may be contained as the dopant element in a concentration of no lessthan 1.3×10¹⁶ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Ca may be contained as the dopant element in a concentration of no lessthan 3.2×10¹⁶ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Cd may be contained as the dopant element in a concentration of no lessthan 6.4×10¹⁵ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Ga may be contained as the dopant element in a concentration of no lessthan 8.1×10¹⁵ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

In may be contained as the dopant element in a concentration of no lessthan 5.1×10¹⁵ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Li may be contained as the dopant element in a concentration of no lessthan 3.2×10¹⁶ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Mg may be contained as the dopant element in a concentration of no lessthan 1.0×10¹⁷ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Zn may be contained as the dopant element in a concentration of no lessthan 1.6×10¹⁶ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a diamondis doped with any of the elements Al, Be, Ca, Cd, Ga, In, Li, Mg and Znby ultra-high-temperature and high-pressure synthesis.

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a diamondis doped with any of elements Al, Be, Ca, Cd, Ga, In, Li, Mg and Zn byion implantation.

Still another aspect of the present invention is a process for producinga p-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Al atoms to the number of C atoms (Al/C) is noless than 0.26 ppm and no more than 10⁴ ppm.

The Al dopant gas may include any of the following four: aluminum (Al)vaporized by introducing solid-state Al into plasma, trimethylaluminum((CH₃)₃Al: TMAl), triethylaluminum ((C₂H₅)₃Al: TEAl) and aluminiumchloride (AlCl₃).

Another aspect of the present invention is a process of producing ap-type diamond semiconductor, which is characterized in that, a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Be atoms to the number of C atoms (Be/C) is noless than 0.13 ppm and no more than 10⁴ ppm.

The Be dopant gas may include beryllium (Be) vaporized by introducingsolid-state Be into plasma.

Another aspect of the present invention is a process of producing ap-type diamond semiconductor, which is characterized in that, a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Ca atoms to the number of C atoms (Ca/C) is noless than 0.32 ppm and no more than 10⁴ ppm.

The Ca dopant gas may include either calcium (Ca) vaporized byintroducing solid-state Ca into plasma, or calcium chloride (CaCl₂).

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Cd atoms to the number of C atoms (Cd/C) is noless than 0.064 ppm and no more than 10⁴ ppm.

The Cd dopant gas may include any of the following three:dimethylcadmium ((CH₃)₂Cd: DMCd), diethylcadmium ((C₂H₅)₂Cd: DECd) andcadmium chloride (CdCl₂).

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Ga atoms to the number of C atoms (Ga/C) is noless than 0.081 ppm and no more than 10⁴ ppm.

The Ga dopant gas may include any of the following three:trimethylgallium ((CH₃)₃Ga: TMGa), triethylgallium ((C₂H₅)₃Ga: TEGa) andgallium chloride (GaCl₃).

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of In atoms to the number of C atoms (In/C) is noless than 0.051 ppm and no more than 10⁴ ppm.

The In dopant gas may include any of the following three:trimethylindium ((CH₃)₃In: TMIn), triethylindium ((C₂H₅)₃In: TEIn) andindium chloride (InCl₃).

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Li atoms to the number of C atoms (Li/C) is noless than 3.0 ppm and no more than 10⁴ ppm.

The Li dopant gas may include any of the following three: methyllithium(CH₃Li), ethyllithium (C₂H₅Li) and propyllithium (C₃H₇Li).

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Mg atoms to the number of C atoms (Mg/C) is noless than 1.0 ppm and no more than 10⁴ ppm.

The dopant gas including Mg may include any of the following three:bis-cyclopentadienyl magnesium ((C₅H₅)₂Mg: Cp₂Mg),bis-methylcyclopentadienyl magnesium ((CH₃C₅H₄)₂Mg: MCp₂Mg) andmagnesium chloride (MgCl₂).

Another aspect of the present invention is a process for producing ap-type diamond semiconductor, which is characterized in that a p-typediamond semiconductor film is grown, by a chemical vapor deposition(CVD) technique, on a diamond substrate from a source gas including, atleast, a gas that includes carbon (C) and a dopant gas in which theratio of the number of Zn atoms to the number of C atoms (Zn/C) is noless than 0.16 ppm and no more than 10⁴ ppm.

The dopant gas including Zn may include any of the following three:dimethylzinc ((CH₃)₂Zn: DMZn), diethylzinc ((C₂H₅)₂Zn: DEZn) and zincchloride (ZnCl₂).

Another aspect of the present invention is an MESFET, which ischaracterized by comprising: a diamond substrate; the p-type diamondsemiconductor layer, produced by any of the aforementioned processes,formed on the diamond substrate; a source electrode and a drainelectrode formed on the p-type diamond semiconductor layer at a distancefrom each other; and a gate electrode formed between the sourceelectrode and the drain electrode on the p-type diamond semiconductorlayer.

Still another aspect of the present invention is an MISFET, which ischaracterized by comprising: a diamond substrate; the p-type diamondsemiconductor layer, produced by any of the aforementioned processes,formed on the diamond substrate; a source electrode and a drainelectrode formed on the p-type diamond semiconductor layer at a distancefrom each other; an insulating layer formed between the metal electrodeson the p-type diamond semiconductor layer: and a gate electrode formedon the insulating layer.

Another aspect of the present invention is an npn-type bipolartransistor, which is characterized by comprising: a diamond substrate; afirst n-type diamond semiconductor layer formed on the diamondsubstrate; the first p-type diamond semiconductor layer produced by anyof the aforementioned processes, and a first metal electrode that areformed on the first n-type diamond semiconductor layer at a distancefrom each other; a second n-type diamond semiconductor layer and asecond metal electrode that are formed on the p-type diamondsemiconductor layer at a distance from each other; and a third metalelectrode formed on the second n-type diamond semiconductor layer.

Another aspect of the present invention is a pnp-type bipolartransistor, which is characterized by comprising: a diamond substrate;the first p-type diamond semiconductor layer, produced by any of theaforementioned processes, formed on the diamond substrate; an n-typediamond semiconductor layer and a first electrode that are formed on thefirst p-type diamond semiconductor layer at a distance from each other;the second p-type diamond semiconductor layer, produced by any of theaforementioned processes and formed on the diamond substrate, and asecond electrode that are formed on the n-type diamond semiconductorlayer at a distance from each other; and a third electrode formed on thesecond p-type diamond semiconductor layer.

Another aspect of the present invention is an LED, which ischaracterized by comprising: a diamond substrate; the p-type diamondsemiconductor layer, produced by any of the aforementioned processes,formed on the diamond substrate; an n-type diamond semiconductor layerand an anode electrode that are formed on the p-type diamondsemiconductor layer at a distance from each other; and a cathodeelectrode formed on the n-type diamond semiconductor layer.

According to the present invention, it becomes possible to produce apractically useful p-type diamond semiconductor having a holeconcentration of no less than 1.0×10¹⁵ cm⁻³ and a dopant atomconcentration of no more than 1.0×10²¹ cm⁻³ at no less than roomtemperature (300K). In addition, it is possible to significantlyincrease the hole concentration of the p-type diamond semiconductorwhile reducing the dopant atom concentration to no more than 1.0×10²¹cm⁻³.

In the present invention, in order to attain the aforementioned fifthobject, an aspect of the present invention is characterized, in aprocess for producing a diamond semiconductor, by comprising: a firstprocess-step of implanting dopant into a diamond by an ion implantationtechnique; a second process-step of forming a protective layer on atleast part of the surface of the ion-implanted diamond; and a thirdprocess-step of firing the ion-implanted diamond protected by theprotective layer under a pressure of no less than 3.5 GPa and at atemperature of no less than 600° C.

Another aspect of the present invention is characterized, in a processfor producing a diamond semiconductor, by comprising: a firstprocess-step of implanting dopant into a diamond by an ion implantationtechnique; and a second process-step of superimposing two of theion-implanted diamond on one another such that at least part of thesurfaces of the ion-implanted diamonds make contact with each other, andthen of firing them under a pressure of no less than 3.5 GPa and at atemperature of no less than 600° C.

The dopant implanted by the ion implantation technique may include atleast one of kinds, B, Al, Ga, In, Zn, Cd, Be, Mg, Ca, P, As, Sb, O, S,Se, Li, Na, and K.

The pressure P (kbar) and the temperature T (K) when the ion-implanteddiamond is fired may be a pressure of no less than 35 kbar and atemperature of no less than 873K which satisfy the relation of theexpression P>7.1+0.027 T.

The protective layer may be any of the following layers: a layer of ametal including at least one of the metals titanium, tungsten, platinum,palladium and molybdenum, a layer of Al_(1-x)Si_(x)O_(1-y)N_(y) (0≦x≦1,0≦y≦1) and a layer comprising many layers of no less than two of them.

The diamond used in the first process-step may be a diamond thin-filmproduced by a CVD technique.

According to the present invention, the provision of high-quality P-typeand N-type diamond semiconductors having high mobility is made possiblyby performing high-temperature and high-pressure annealing treatment onthe ion-implanted diamond after the surface is protected by a protectivelayer. In addition, because a semiconductor having a sufficiently highfree-electron and hole concentration at room temperature is obtained, itis possible to implement a high-frequency high-power transistor withhigh performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating the structure of a diamondsemiconductor element according to a first embodiment of the presentinvention.

FIG. 1B is a diagram illustrating the structure of a diamondsemiconductor element according to a first embodiment of the presentinvention.

FIG. 2 is a diagram illustrating the structure of a diamondsemiconductor element according to a second embodiment of the presentinvention.

FIG. 3A is a diagram illustrating a process-step in producing thediamond semiconductor element of the first embodiment.

FIG. 3B is a diagram illustrating a process-step in producing thediamond semiconductor element of the first embodiment.

FIG. 3C is a diagram illustrating a process-step in producing thediamond semiconductor element of the first embodiment.

FIG. 3D is a diagram illustrating a process-step in producing thediamond semiconductor element of the first embodiment.

FIG. 4A shows transistor characteristics of the diamond semiconductorelement according to the first embodiment of the present invention.

FIG. 4B shows transistor characteristics of the diamond semiconductorelement according to the first embodiment of the present invention.

FIG. 5A is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 5B is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 5C is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 6 is a diagram illustrating the structure of a diamondsemiconductor element according to the present invention in which adiamond substrate is removed.

FIG. 7A is a diagram illustrating a process-step in producing a diamondsemiconductor element according to an embodiment of the presentinvention.

FIG. 7B is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7C is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7D is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7E is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7F is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7G is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7H is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7I is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 7J is a diagram illustrating a process-step in producing a diamondsemiconductor element according to the embodiment of the presentinvention.

FIG. 8A is a graph showing the characteristics of a diamond field-effecttransistor having a gate length of 0.2 μm using a diamond semiconductorelement according to an embodiment of the present invention, which is agraph showing drain current-voltage characteristics.

FIG. 8B is a graph showing the characteristics of a diamond field-effecttransistor having a gate length of 0.2 μm using a diamond semiconductorelement according to an embodiment of the present invention, which is agraph showing the dependence of the transconductance g_(m) on the gatevoltage V_(G) (transfer characteristics).

FIG. 8C is a graph showing the characteristics of a diamond field-effecttransistor having a gate length of 0.2 μm using a diamond semiconductorelement according to an embodiment of the present invention, which is agraph showing the dependence of the power gain U on the frequency f.

FIG. 9A is a graph showing the relationship between a lower source-gatedistance d_(SGB) and the maximum oscillation frequency f_(max).

FIG. 9B is a graph showing the relationship between an upper source-gatedistance d_(SGT) and the maximum oscillation frequency f_(max).

FIG. 10A is a graph showing the relationship between a gate-lower draindistance d_(GDB) and the drain breakdown voltage V_(BR).

FIG. 10B is a graph showing the relationship between a gate-upper draindistance d_(GDT) and the maximum oscillation frequency f_(max).

FIG. 11A is a graph showing the relationship between the thicknesst_(SB) of a lower source electrode 2-6SB and the maximum oscillationfrequency f_(max).

FIG. 11B is a graph showing the relationship between the thicknesst_(ST) of an upper source electrode 2-6ST and the maximum oscillationfrequency f_(max).

FIG. 12A is a graph showing the relationship between the thicknesst_(DB) of a lower drain electrode 2DB and the maximum oscillationfrequency f_(max).

FIG. 12B is a graph showing the relationship between the thicknesst_(DT) of an upper drain electrode 2DB and the maximum oscillationfrequency f_(max).

FIG. 13A is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 13B is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 13C is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 13D is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 13E is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 13F is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 13G is a diagram illustrating a process-step in producing a diamondsemiconductor element according to conventional art.

FIG. 14A is a graph showing the drain current-voltage characteristicswhich are characteristics of a transistor using a diamond semiconductorelement according to conventional art.

FIG. 14B is a graph showing the dependence of the transconductance g_(m)on the gate voltage V_(G) (transfer characteristics) which ischaracteristics of a transistor using a diamond semiconductor elementaccording to conventional art.

FIG. 14C is a graph showing characteristics of a transistor using adiamond semiconductor element according to conventional art, which showsthe dependence of the power gain U on the frequency f.

FIG. 15A is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 1 of the present invention.

FIG. 15B is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 1 of the present invention.

FIG. 15C is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 1 of the present invention.

FIG. 15D is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 1 of the present invention.

FIG. 16A is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 2 of the present invention.

FIG. 16B is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 2 of the present invention.

FIG. 16C is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 2 of the present invention.

FIG. 16D is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 2 of the present invention.

FIG. 17A is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 3 of the present invention.

FIG. 17B is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 3 of the present invention.

FIG. 17C is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 3 of the present invention.

FIG. 17D is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 3 of the present invention.

FIG. 17E is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 3 of the present invention.

FIG. 17F is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 3 of the present invention.

FIG. 18A is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 4 of the present invention.

FIG. 18B is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 4 of the present invention.

FIG. 18C is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 4 of the present invention.

FIG. 18D is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 4 of the present invention.

FIG. 18E is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 4 of the present invention.

FIG. 18F is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 4 of the present invention.

FIG. 19A is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 5 of the present invention.

FIG. 19B is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 5 of the present invention.

FIG. 19C is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 5 of the present invention.

FIG. 19D is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 5 of the present invention.

FIG. 20A is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 6 of the present invention.

FIG. 20B is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 6 of the present invention.

FIG. 20C is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 6 of the present invention.

FIG. 20D is a diagram illustrating a process-step in producing a diamondthin-film according to embodiment example 6 of the present invention.

FIG. 21 is a graph showing the region in which diamond and graphite arein a stable state, according to an embodiment of the present invention.

FIG. 22A is a diagram illustrating a process for producing a diamondthin-film according to a conventional method.

FIG. 22B is a diagram illustrating a process for producing a diamondthin-film according to a conventional method.

FIG. 23A is a graph for a comparison of the current-voltagecharacteristics of a field-effect transistor produced on a diamondsingle-crystal thin-film between before and after high-pressure andhigh-temperature annealing, according to embodiment example 1 of thepresent invention.

FIG. 23B is a graph for a comparison of the current-voltagecharacteristics of a field-effect transistor produced on a diamondsingle-crystal thin-film between before and after high-pressure andhigh-temperature annealing, according to embodiment example 1 of thepresent invention.

FIG. 24 is a graph showing the relationship between the concentration ofdopant atoms in a source gas and the concentration of dopant atoms in ap-type diamond semiconductor, according to an embodiment of the presentinvention.

FIG. 25 is a graph showing temperature dependence of a holeconcentration for each Al atom concentration in a p-type diamondsemiconductor according to embodiment 4 of the present invention.

FIG. 26 is a graph showing temperature dependence of a holeconcentration for each Be atom concentration in a p-type diamondsemiconductor according to embodiment 5 of the present invention.

FIG. 27 is a graph showing temperature dependence of a holeconcentration for each Ca atom concentration in a p-type diamondsemiconductor according to embodiment 6 of the present invention.

FIG. 28 is a graph showing temperature dependence of a holeconcentration for each Cd atom concentration in a p-type diamondsemiconductor according to embodiment 7 of the present invention.

FIG. 29 is a graph showing temperature dependence of a holeconcentration for each Ga atom concentration in a p-type diamondsemiconductor according to embodiment 8 of the present invention.

FIG. 30 is a graph showing temperature dependence of a holeconcentration for each In atom concentration in a p-type diamondsemiconductor according to embodiment 9 of the present invention.

FIG. 31 is a graph showing temperature dependence of a holeconcentration for each Mg atom concentration in a p-type diamondsemiconductor according to embodiment 11 of the present invention.

FIG. 32 is a graph showing temperature dependence of a holeconcentration for each Zn atom concentration in a p-type diamondsemiconductor according to embodiment 12 of the present invention.

FIG. 33 is a sectional schematic diagram illustrating MESFET(metal-semiconductor field-effect transistor) according to embodiment 15of the present invention.

FIG. 34 is a sectional schematic diagram illustrating MISFET(metal-insulating film-semiconductor field-effect transistor) accordingto embodiment 16 of the present invention.

FIG. 35 is a sectional schematic diagram illustrating an npn-typebipolar transistor according to embodiment 17 of the present invention.

FIG. 36 is a sectional schematic diagram illustrating an pnp-typebipolar transistor according to embodiment 18 of the present invention.

FIG. 37 is a sectional schematic diagram illustrating a light emittingdiode (LED) according to embodiment 19 of the present invention.

FIG. 38 is a graph showing temperature dependence of a holeconcentration for each B atom concentration in a conventional p-typediamond semiconductor.

FIG. 39 is a graph showing temperature dependence of a holeconcentration for each Li atom concentration in a p-type diamondsemiconductor according to embodiment 10 of the present invention.

FIG. 40A is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 40B is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 40C is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 40D is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 40E is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 40F is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 40G is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 1 of the present invention.

FIG. 41A is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 2 of the present invention.

FIG. 41B is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 2 of the present invention.

FIG. 41C is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 2 of the present invention.

FIG. 41D is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 2 of the present invention.

FIG. 41E is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 2 of the present invention.

FIG. 41F is a diagram illustrating a process-step in producing a diamondsemiconductor according to embodiment 2 of the present invention.

FIG. 42 is a graph showing cathode luminescence (CL) spectra(measurement temperature: 10K) before and after high-temperaturehigh-pressure annealing is performed on a diamond thin-film implantedwith ions of boron (B) as dopant.

FIG. 43A is a diagram illustrating a process-step in producing a diamondsemiconductor using an ion implantation technique according toconventional art.

FIG. 43B is a diagram illustrating a process-step in producing a diamondsemiconductor using an ion implantation technique according toconventional art.

FIG. 43C is a diagram illustrating a process-step in producing a diamondsemiconductor using an ion implantation technique according toconventional art.

FIG. 43D is a diagram illustrating a process-step in producing a diamondsemiconductor using an ion implantation technique according toconventional art.

FIG. 43E is a diagram illustrating a process-step in producing a diamondsemiconductor using an ion implantation technique according toconventional art.

DETAILED DESCRIPTION OF THE INVENTION

In order to attain the first object, a diamond semiconductor elementaccording to the present invention is characterized in that each surfaceorientation of a single-crystal diamond substrate, a single-crystaldiamond thin-film, and a hole channel or electron channel, and thedirection of forming a gate electrode are defined as directions peculiarto the present invention. The structure of a diamond semiconductorelement according to the present invention in order to attain the firstobject and a process for producing the diamond semiconductor elementwill be described below in detail.

Embodiment 1

FIGS. 1A and 1B are schematic diagrams of a diamond semiconductorelement according to embodiment 1 of the present invention. Asillustrated in the sectional view in FIG. 1B, a single-crystal diamondthin-film 1-2 is formed on a single-crystal diamond substrate 1-1. Atwo-dimensional hole or electron channel 1-3 is formed in thesingle-crystal diamond thin-film 1-2. Then, an angle formed by thesurface orientation of the single-crystal diamond substrate 1-1 and thecrystal axis [001] direction of the single-crystal diamond substrate 1-1is defined as αs, an angle formed by the surface orientation of thesingle-crystal diamond thin-film 1-2 and the crystal axis [001]direction of the single-crystal diamond thin-film 1-2 is defined as αd,and an angle formed by the surface orientation of the channel 1-3 andthe crystal axis [001] direction of the single-crystal diamond thin-film1-2 is defined as αc. As illustrated in the top view in FIG. 1A, asource electrode 1-4, a gate electrode 1-5 and a drain electrode 1-6 areformed on the surface of the single-crystal diamond thin-film 1-2. Thechannel 1-3 formed in the single-crystal diamond thin-film 1-2 isindicated with a dotted line. An angle formed by the longitudinaldirection of the gate electrode 1-5, which is{right arrow over (g)},  [Expression 5]and the crystal axis [110] of the single-crystal diamond thin-film isdefined as β. A description of αs, αd, αc and β will be given later.Next, a process of producing the diamond semiconductor element will bedescribed.

FIGS. 3A to 3D are diagrams illustrating the process-steps in producingthe diamond semiconductor element according to the present invention. Asshown in FIG. 3A, first, a single-crystal diamond substrate 1-1 havingsurface orientation oriented mainly in the direction of the crystal axisis prepared.

Then, as shown in FIG. 3B, surface grinding is performed such that thesurface direction of the surface of the single-crystal diamond substrate1-1, which is{right arrow over (s)},  [Expression 6]is inclined αs (°) from the [001] direction of the crystal axis of thesingle-crystal diamond substrate 1-1. The surface grinding may becarried out by the following procedure, for example. First, the inclinedangle from the [001] surface orientation of the substrate surface ismeasured in advance by X-ray diffraction measurement. Then, thesingle-crystal diamond substrate 1-1 is bonded to a sample tray withwax. Then, an iron flat plate is coated with diamond abrasive grain(particle diameter of from 0.3 μm to 1 μm) together with oil. The sampletray on which the single-crystal diamond substrate 1-1 is bonded isplaced on the iron flat plate, and is then fixed, and then the iron flatplate is rotated for grinding. After grinding has been performed for afixed time-period, the inclined angle is measured by X-ray diffractionmeasurement. The above-described grinding and the X-ray diffractionmeasurement are repeated until the desired angle αs is obtained. Inother words, as shown in FIG. 3B, the substrate is cut away until thesurface orientation of the single-crystal diamond substrate reaches apredetermined as by grinding. The face inclined αs (°) may be only theface on which a single-crystal diamond thin-film will be grown in thenext process-step described later.

Next, as shown in FIG. 3C, crystal growth is performed on thesingle-crystal diamond substrate 1-1 having the surface orientationangle αs to form a single-crystal diamond thin-film 1-2 thereon. Thiscrystal growth of the thin-film may be carried out by, for example, amicrowave plasma CVD technique. More specifically, a methane gas and ahydrogen gas (the flow ratio of methane gas is 1%) are fed as a sourcegas into a reaction tube in which the single-crystal diamond substrate1-1 is placed. The degree of vacuum in the reaction tube is set at 50Torr, and a microwave is applied at 2.45 GHz and 1.3 kW to generateplasma in the reaction tube. The temperature of the single-crystaldiamond substrate 1-1 is set at 700 degrees (all this is condition 1).The surface orientation of the surface of the single-crystal diamondthin-film 1-2, which is{right arrow over (d)},  [Expression 7]is able to be formed at an inclination of αd (°) from the [001]direction of the crystal axis of the single-crystal diamond thin-film1-2, as in the case of the surface of the single-crystal diamondsubstrate 1-1.

A two-dimensional hole or electron channel 1-3 is formed inside thesingle-crystal diamond thin-film 1-2 along the surface of thesingle-crystal diamond thin-film 1-2. The channel 1-3 is formed by, forexample, a microwave plasma CVD technique as in the case of theaforementioned single-crystal diamond thin-film 1-2. Specifically, theconditions of the temperature and the degree of vacuum in the reactiontube and the microwave are the same, but the condition of the source gasis changed so that only a hydrogen gas is used (condition 2). Under thiscondition 2, plasma is applied for 15 minutes to the surface of thesingle-crystal diamond thin-film 1-2 under the growing process, therebyforming the channel 1-3. Then, the previous condition (condition 1) isrestored, the single-crystal diamond thin-film 1-2 is formed again,thereby forming the channel 1-3 inside the single-crystal diamondthin-film 1-2.

Note that the aforementioned condition 1 and condition 2 apply to anexample when a hole channel is used. When an electron channel is used toconstitute a diamond semiconductor element, it is possible to usehydrogen, a methane gas, a phosphine (PH3) gas as a source gas(condition 3). In this case, the single-crystal diamond thin-film 1-2 isformed initially under condition 1, then the condition is changed tocondition 3 to form an electron channel 1-3, and then the condition ischanged back to condition 1 to form the single-crystal diamond thin-film1-2 again. The surface orientation of the forming face of thetwo-dimensional channel 1-3, which is{right arrow over (c)},  [Expression 8]is able to be formed at an inclination of αc (°) from the [001]direction of the crystal axis of the single-crystal diamond thin-film1-2.

Next, as shown in FIG. 3D, a source electrode 1-4, a gate electrode 1-5and a drain electrode 1-6 are formed on the surface of thesingle-crystal diamond thin-film 1-2. When the surface of thesingle-crystal diamond thin-film 1-2 is viewed from above, the channel1-3 exists directly below the source electrode 1-4 and the drainelectrode 1-6 (the channel 1-3 is indicated with the dotted line in thetop view in FIG. 3D). The gate electrode 1-5 is positioned midwaybetween the source electrode 1-4 and the drain electrode 1-6. When thesurface of the single-crystal diamond thin-film 1-2 is viewed fromabove, the gate electrode 1-5 is positioned so as to intersect with thechannel 1-3. The gate electrode 1-5 is produced such that thelongitudinal direction of the gate electrode 1-5 above the channel 1-3,which is{right arrow over (s)},  [Expression 9]becomes the [001] direction of the crystal axis of the single-crystaldiamond thin-film or a direction inclining β(degrees) from the [001]direction of the crystal axis, as shown in FIG. 3D. By way of each ofthe above process-steps, the field-effect transistor based on thediamond semiconductor element structure according to the presentinvention is completed.

At this stage, the relationship between αs, αd and αc will be describedin further detail. First, αs is determined by the process-step ofgrinding the aforementioned single-crystal diamond substrate 1-1, andthen without further processing, the single-crystal diamond thin-filmlayer 1-2 and the channel 1-3 are formed, resulting in αs=αd=αc.However, without limiting to this condition, each of the angles can beindependently controlled. For example, after the single-crystal diamondthin-film 1-2 is formed and before the electrodes are formed, thesurface of the single-crystal diamond thin-film 1-2 is ground forinclination. Thereby, it is possible to make the angle αd and the angleαs different from each other. Also, immediately before the channel 1-3is formed, the substrate is temporarily removed from the reaction tube,and then the surface of the single-crystal diamond thin-film which hasbeen formed up to that time is ground for inclination. Then, the channel1-3 is formed. Thereby, it is possible to control αs and αcindependently of each other.

FIGS. 4A and 4B are graphs showing the characteristics of thefield-effect transistor produced by the above-described process-stepsaccording to the present invention. FIG. 4A is a graph showing therelationship between the maximum transconductance gmmax and the angles(αs, αd, αc) inclined from the [001] direction of orientation of eachsurface. Here, the angles αs, αd, αc are equally formed aresimultaneously changed. The transconductance is a basic parameterrepresenting the performance of the field-effect transistor. In general,the existence of crystal defects is able to be evaluated by therepresentative device performance gm. If the gm is high, fewer crystaldefects can be evaluated. Note that the value of β in FIG. 4B, describedlater, is zero degrees.

In a transistor produced by conventional art, the surface orientation ofthe single-crystal diamond substrate or the like points precisely in the[001] direction. In other words, the state of αs=0°, αc=0° and αd=0°results. In this case, the gmmax is about 40 mS/mm to about 80 mS/mm.

As seen from FIG. 4A, with an increase of αs, αc and αd, the gmmaxsharply increases, and under the condition that αs=0.05°, αc=0.05° andαd=0.05°, gmmax=310 mS/mm to 390 mS/mm results. When αs, αc and αd arewithin the range of 0.05° to 1.1°, the gmmax is 300 mS/mm or more. Onthe other hand, when αs, αc and αd exceed 1.1°, gmmax sharply decreases.In this manner, the setting of αs, αc and αd within the range of 0.05°to 1.1° makes it possible to obtain a significantly higher gmmax thanthat in the case of conventional art. As described earlier, the shiftingof the surface orientation of the single-crystal diamond substrate orthe like slightly from the [001] direction makes it possible toremarkably suppress the occurrence of the crystal defects peculiar to adiamond, thus providing transconductance of a large field-effecttransistor.

FIG. 4B is a graph showing the relationship between the maximumtransconductance gmmax and the inclination of the angle β from the [110]direction with respect to the longitudinal direction of the gateelectrode. In this case, αs=αc=αd=0.05° is established.

In the conventional art, the [100] direction is used for thelongitudinal direction of the gate electrode. In other words, when thelongitudinal direction of the gate electrode is pointed in the [100]direction, it corresponds to the state of β=±45° in FIG. 4B. Under thiscondition that β=±45° which is the conventional art, the gmmax is 25mS/mm at the most.

In the diamond semiconductor element according to the present invention,when 13 is in the range from −30° to 30°, the gmmax is 100 mS/mm ormore, and in β=0° (corresponding to g=[110]), the gmmax reaches 200mS/mm. In this way, only after the direction in which the gate electrode1-5 has been formed is optimized within the above-described range, it ispossible to produce a field-effect transistor using a diamondsemiconductor having a transconductance at a practical level.

The foregoing has described the case when the channel 1-3 is formedinside the single-crystal diamond thin-film 1-2. However, even in astructure in which the single-crystal diamond thin-film is not formedagain after the process-step of forming the channel 1-3, and the sourceelectrode 1-4, the gate electrode 1-5 and the drain electrode 1-6 areformed directly on the surface of the channel 1-3, what results can beoperated as a semiconductor element. In other words, a structure withoutthe single-crystal diamond thin-film 1-2 formed above the channel 1-3 inFIG. 1B is also allowable. In this case, if the angle αc formed betweenthe surface orientation of the surface of the channel 1-3 and thecrystal axis [001] direction of the single-crystal diamond thin-film1-2, and the angle αs formed between the surface orientation of thesingle-crystal diamond substrate 1-1 and the crystal axis [001]direction of the single-crystal diamond substrate 1-1 are each setwithin the aforementioned range, the same effect of suppressing theoccurrence of crystal defects is obtained by a more simple structure.Thus, it is needless to say that, the structure of the channel 1-3 isnot limited to being placed exactly halfway in the thickness directionof the single-crystal diamond thin-film layer 1-2 as illustrated in FIG.1B.

Also, it is possible to provide a structure without the single-crystaldiamond substrate 1-1 as a result of the removal of the single-crystaldiamond substrate 1-1 in FIG. 1 before the source electrode 1-4, thegate electrode 1-5 and the drain electrode 1-6 are formed. That is, theeffects peculiar to the present invention can be provided by thestructure, as illustrated in FIG. 6, constituted of the single-crystaldiamond thin-film 1-2, channel 1-3, source electrode 1-4, gate electrode1-5 and the drain electrode 1-6 after the single-crystal diamondsubstrate 1-1 has been removed from FIG. 1. In this case, thesingle-crystal diamond thin-film 1-2 has a relatively thickerfilm-thickness such that the film alone can structurally support itselfalone. The removal of the single-crystal diamond substrate 1-1 may beperformed by, for example, grinding.

Next, the definition of the surface orientation used in thisspecification will be described. It is needless to say that the [110]direction and the [100] direction are the definition of the directionwhen a principal direction of the plane of the single-crystal diamondsubstrate 1-1 or the principal direction of the surface of thesingle-crystal diamond thin-film 1-2 is designated as the [001]direction. A diamond crystallographically has the lattice system, whichis a cubic lattice (cubic crystal). Typically, the crystal plane has theproperty of being apt to appear as the number of atom bonds necessary tobe broken when the plane is formed becomes fewer. For this reason, inthe case of a diamond, the surface orientations apt to appear are [001],[111] and [011]. As described above, since the diamond is of a cubiccrystal system, [001], [100], [010] and the like are physicallyequivalent. Mathematically, [100], [−100], [010], [0-10], [00-1] arealso present. However, they are all equivalent to [001] because of thecrystal symmetry (for details see non-patent document 2). In thisspecification, the principal direction of the single-crystal diamondsubstrate 1-1 plane or the principal direction of the single-crystaldiamond thin-film 1-2 surface is described as the [001] direction. Notethat the “principal direction” means the surface orientation which ismost apt to appear as described above.

When the principal direction of the single-crystal diamond substrate 1-1plane or the principal direction of the single-crystal diamond thin-film1-2 surface is defined as the direction, the [100] direction, which is apreferable direction of the longitudinal direction of the gate electrode1-5, is crystallographically equivalent to the [010] direction.Accordingly, it goes without saying that the description of the [100]direction in this specification includes the [010] direction. Note that[100], [−100], [010], [0-10], [00-1] are equivalent as describedearlier, and they are generically referred to as the [100] directionwhich is a preferable direction of the longitudinal direction of thegate electrode 1-5. It is to be noted that the [110] direction also is ageneric name.

In the process-steps shown in FIGS. 3A to 3D, the single-crystal diamondsubstrate 1-1 is used. However, even when the single-crystal diamondsubstrate 1-1 is not used, as long as the surface orientation of thesingle-crystal diamond thin-film 1-2 or the surface orientation of thechannel 1-3 plane falls within each of the αd, αc angle ranges describedin FIG. 4, it is needless to say that the effect of increasing thetransconductance is produced by the same token. When the single-crystaldiamond substrate 1-1 is not used, Ir (iridium metal), MgO, Si, forexample, may be used as a substrate. What is required is for thesubstrate to allow the single-crystal diamond thin-film to be formedthereon.

Embodiment 2

FIG. 2 is a diagram illustrating the structure of a diamondsemiconductor element according to embodiment 2 of the presentinvention. The structure of embodiment 2 is suitable for high poweroperation. In embodiment 2, a gate electrode is arranged in the form ofa sport ground track. A single-crystal diamond thin-film 1-12 is formedon a single-crystal diamond substrate 1-11 and a two-dimensional hole orelectron channel 1-13 is formed inside the single-crystal diamondthin-film. As shown in FIG. 2, the channel has an oval shape. Then, asource electrode 1-14, a gate electrode 1-15 and a drain electrode 1-16are formed on the surface of the single-crystal diamond thin-film 1-12above the channel 1-13. The gate electrode 1-15 and the source electrode1-14 are arranged outward in order so as to encircle the periphery ofthe oval-shaped drain electrode 1-16 placed innermost.

FIG. 2 does not clearly show, but in embodiment 2, the surfaceorientations of the single-crystal diamond substrate 1-11, thesingle-crystal diamond thin-film 1-12 and the channel 1-13 are also setrespectively at αs, αd, αc falling within the same range as in the caseof embodiment 1. Each of the electrodes is placed such that in thelinear area, except for the two arc portions corresponding to the twoends of the oval shape, the longitudinal direction of the gate electrode1-15 is inclined β degrees from the crystal axis [110] direction. It isneedless to say that the effect of increasing the transconductance isproduced as in the case shown in FIG. 4.

As described above in detail, by shifting the surface orientation of thesingle-crystal diamond thin-film, the single-crystal diamond substrateor the channel-forming plane slightly from the [001] direction, it ispossible to provide the effect peculiar to the present invention inwhich the occurrence of crystal defects peculiar to a diamond issignificantly suppressed. Transconductance of a significantly largefield-effect transistor is provided, resulting in the realization of apractically useful diamond semiconductor element.

Next, a diamond semiconductor element and a process for producing thediamond semiconductor element according to the present invention inorder to attain the second object will be described in detail.

Embodiment 3

FIGS. 7A to 7J illustrate process-steps in producing a diamondsemiconductor element according to an embodiment of the presentinvention. A thin-film 2-1, which is a diamond single-crystal and has atwo-dimensional hole channel extending parallel to the surface,(hereinafter referred to as “diamond single-crystal thin-film 2-1”) isprepared (FIG. 7A). Au is evaporated to a thickness of 0.1 μm onto theentire surface of the diamond single-crystal thin-film 2-1 to form an Authin-film 2-2B (FIG. 7B). Then, the Au thin-film 2-2B is coated with aresist 2-3 with a width of 20 μm in the area in which a gate electrodewill be provided (FIG. 7C), and from the above, an Au thin-film 2-2T isevaporated onto the entire surface of the sample (FIG. 7D). At thispoint, the thickness of the Au thin-film 2-2T is 0.4 μm.

Next, liftoff is performed on the resist 2-3 to remove the resist 2-3and part of the Au thin-film 2-2T evaporated onto the resist 2-3 so asto form an aperture 2-7 in the Au thin-film 2-2T (FIG. 7E). For asuccessful liftoff of the resist 2-3, the distance in the directionvertical to the longitudinal direction of the resist 2-3 and in thedirection parallel to the plane of the diamond single-crystal thin-film2-1 is required to be longer than the thickness of the Au thin-film 2-2Tin the direction vertical to the plane of the diamond single-crystalthin-film 2-1. Then, the full surface of the sample is coated withresist 2-4 (FIG. 7F). Exposure and development are performed on an areahaving a width of 0.05 μm, in which a gate electrode will be formed,within the area of the resist 2-4 in contact with the Au thin-film 2-2B,to remove part of the resist 2-4 and to form an aperture 2-8 in the Authin-film 2-2B (FIG. 7G).

Then, an etching solution is used to etch the part of the Au thin-film2-2B exposed by the aperture 2-8 (FIG. 7H). Because etching takes placein an isotropic manner, the area of the Au thin-film 2-2B under theresist 2-4 is also etched to produce an undercut. The etching isterminated before reaching the Au thin-film 2-2T.

Next, Al is evaporated onto the full surface of the sample (FIG. 7I).Part of the Al is evaporated onto the diamond single-crystal thin-film2-1 from the aperture 2-8 in the Au thin-film 2-2B so as to form an Althin-film 2-5G. On the other hand, the remaining major portion of the Alis evaporated onto the resist 2-4 so as to form an Al thin-film 2-5.Then, liftoff is performed on the resist 2-4 to remove the resist 2-4and the Al thin-film 2-5 placed on the resist 2-4 (FIG. 7J).

Here, one of the Au thin-films 2-2B is defined as a lower sourceelectrode 2-6SB, and the other is defined as a lower drain electrode2-6DB. The Au thin-film 2-2T on the lower source electrode 2-6SB isdefined as an upper source electrode 2-6ST, and the Au thin-film 2-2T onthe lower drain electrode 2-6DB is defined as an upper drain electrode2-6DT. It should be noted that this specification describes the lowersource electrode and the upper source electrode as two separate ones,but the lower source electrode 2-6SB and the upper source electrode2-6ST together function as a single source electrode in upper and lowercombination. Thus the lower source electrode 2-6SB and the upper sourceelectrode 2-6ST may be considered to constitute a single sourceelectrode. Likewise, the lower drain electrode and the upper drainelectrode may be considered to constitute a single drain electrode.Also, the Al thin-film 2-5G is defined as a gate electrode 2-7G.

In the diamond semiconductor element according to an embodiment of thepresent invention, the diamond single-crystal thin-film 2-1 may be ofeither single-crystal or polycrystal, and the channel formed in thediamond single-crystal thin-film 2-1 may be for either an electron or ahole.

Here, the gate length is defined as d_(G). The lower source to gatedistance between the end of the lower source electrode 2-6SB in contactwith the surface of the diamond single-crystal thin-film 2-1 and the endof the gate electrode 2-7G close to the source is defined as d_(SGB).The gate to lower drain distance between the end of the lower drainelectrode 2-6DB in contact with the surface of the diamondsingle-crystal thin-film 2-1 and the end of the gate electrode 2-7Gclose to the drain is defined as d_(GDB). The upper source to gatedistance between the end of the upper source electrode 2-6ST and the endof the gate electrode 2-7G close to the source is defined as d_(SGT).The gate to upper drain distance between the end of the upper drainelectrode 2-6DT and the end of the gate electrode 2-7G close to thedrain is defined as d_(GDT).

The thickness of the entire source electrode is defined as t_(SB), inwhich the thickness of the lower source electrode 2-6SB is defined ast_(SB), and the thickness of the upper source electrode 2-6ST is definedas t_(ST). The thickness of the entire drain electrode is defined ast_(D), in which the thickness of the lower drain electrode 2-6DB isdefined as t_(DB), and the thickness of the upper drain electrode 2-6DTis defined as t_(DT).

In this manner, the source electrode and the drain electrode are dividedinto a layer to be etched using an etching solution and a layer subjectto liftoff using resist. As a result, the inverted mesa portion of theelectrode is reduced in size. This makes it possible to shorten thedistance between the source electrode and the gate electrode to increasethe maximum oscillation frequency f_(max), and also to increase thethickness of the source electrode and the drain electrode to bring thedrop in voltage down to a small value.

In the embodiment, the source electrode and the drain electrode are eachconstituted of a metal film made up of two layers, the Au thin-film 2-2Twhich is an upper metal film and the Au thin-film 2-2B which is a lowermetal film, but the source electrode and the drain electrode may be madeup of a number of layers as long as a reduction in the thickness of themetal film in contact with the diamond single-crystal thin-film 2-1 ispossible.

In the embodiment, the lower source electrode 2-6SB and the lower drainelectrode 2-6DB have the ends close to the gate electrode 2-7G formed inan inverted mesa shape. However, a matter of importance for the presentinvention is to independently form the layer for “reducing thesource-gate electrode distance and the gate-drain electrode distance”,and the layer for “increasing the thickness of the source electrode andthe thickness of the drain electrode”. For this reason, the structure ofthe ends of the lower source electrode 2-6SB and the lower drainelectrode 2-6DB closer to the gate electrode 2-7G is not limited to theinverted mesa structure.

FIGS. 8A to 8C show the characteristics of a diamond field-effecttransistor using the diamond semiconductor element according to anembodiment of the present invention. The diamond semiconductor elementused here has the dimensions d_(G)=0.2 μm, d_(SGB)=d_(GDB)=0.1 μm,d_(SGT)=d_(GDT)=1 μm, t_(SB)=t_(DB)=0.05 μm, and t_(ST)=_(DT)=10 μM.

FIG. 8A shows the drain current-voltage characteristics of the diamondfield-effect transistor according to the embodiment. The maximum draincurrent I_(Dmax) is 0.35 A/mm in a conventional type, but reaches 1 A/mmin the embodiment. FIG. 8B shows the dependence of the transconductanceg_(m) on the gate voltage V_(G) (transfer characteristics) in thediamond field-effect transistor according to the embodiment. The maximumtransconductance g_(mmax) is 150 mS/mm in a conventional type, butreaches 520 mS/mm in the embodiment. FIG. 8C shows the dependence of thepower gain U on the frequency f in the diamond field-effect transistoraccording to the embodiment. As shown in FIG. 8C, the maximumoscillation frequency f_(max) is 81 GHz in a conventional type, butreaches 310 GHz in the embodiment.

In this manner, the diamond semiconductor element according to anembodiment of the present invention is greatly improved in thefield-effect transistor characteristics as compared with that in aconventional diamond semiconductor element, and makes possible a diamondtransistor with a practical level.

A change in characteristics caused by the dimensions of a diamondtransistor according to an embodiment of the present invention isdescribed in detail with reference to FIG. 7J. When the shape of thegate electrode 2-7G is the so-called T-type gate (also calledmushroom-type gate) or the like, the distance of the portion of the gateelectrode 2-7G making contact with the surface of the diamondsingle-crystal thin-film is defined as gate length d_(G). In this case,the lower source-gate distance d_(SGB) and the upper source-gatedistance d_(SGT) are respectively defined as the distance from the endof the gate electrode 2-7G which is closer to the source and makescontact with the surface of the diamond single-crystal thin-film 2-1, tothe lower source electrode 2-6SB and the upper source electrode 2-6ST.The gate-lower drain distance d_(GDB) and the gate-upper drain distanced_(GDT) are respectively defined as the distance from the end of thegate electrode 2-7G which is closer to the drain and makes in contactwith the surface of the diamond single-crystal thin-film 2-1, to thelower drain electrode 2-6DB and the upper drain electrode 2-6DT. Thediamond semiconductor element used in FIGS. 9A, 9B to 12A and 12B isassumed to have the dimensions of d_(G)=0.2 μm, d_(SGB)=d_(GDB)=0.1 μm,d_(SGT)=10 μm, d_(GDT)=50 μm, t_(SB)=t_(DB)=0.05 μm, andt_(ST)=t_(DT)=10 μm, except for the dimensions of parts used asvariables.

FIG. 9A shows the relationship between the lower source-gate distanced_(SGB) and the maximum oscillation frequency f_(max). With an increasein the d_(SGB), the f_(max) decreases, but when the d_(SGB) increasesfrom 0.1 μm to 10 μm, the decrease in f_(max) is confined to from 250GHz to 100 GHz, thus making it possible to provide satisfactorycharacteristics as compared with the case in the conventional structurewhere the f_(max) is 81 GHz.

FIG. 9B shows the relationship between the upper source-gate distanced_(SGT) and the maximum oscillation frequency f_(max). With an increasein the d_(SGT), the f_(max) decreases, but when the d_(SGB) increasesfrom 0.07 μm to 30 μm, the decrease in the f_(max) is confined to from250 GHz to 120 GHz, thus making it possible to provide satisfactorycharacteristics as compared with the case in the conventional structurewhere the f_(max) is 81 GHz.

FIG. 10A shows the relationship between the gate-lower drain distanced_(GDB) and the drain breakdown voltage V_(BR). With an increase in thed_(GDB), the V_(BR) increases, but the V_(BR) sharply increases when thed_(GDB) is in about from 0.05 μm to about 0.1 μm, and then the V_(BR)becomes 100V or more when the d_(GDB) is in a region of 0.1 μm or more,thus making it possible to provide satisfactory characteristics ascompared with the case in the conventional structure where the V_(BR) is45V. However, when the d_(GDB) exceeds 50 μm, the f_(max) decreases.

FIG. 10B shows the relationship between the gate-upper drain distanced_(GDT) and the maximum oscillation frequency f_(max). With an increasein the d_(GDT), the f_(max) increases until the d_(GDT) becomes 3 μm,but after the d_(GDT) exceeds 3 μm, the f_(max) decreases. In FIG. 10B,when the d_(GDT) is 30 μm, the f_(max) shows 140 GHz, thus making itpossible to provide satisfactory characteristics as compared with thecase in the conventional structure where the f_(max) is 81 GHz.

FIG. 11A shows the relationship between the thickness t_(SB) of thelower source electrode 2-6SB and the maximum oscillation frequencyf_(max). With an increase in the t_(SB), the f_(max) decreases, but whenthe t_(SB) increases from 0.01 μm to 0.2 μm, the decrease in the f_(max)is confined to from 260 GHz to 130 GHz, thus making it possible toprovide satisfactory characteristics as compared with the case in theconventional structure where the f_(max) is 81 GHz.

FIG. 11B shows the relationship between the thickness t_(ST) of theupper source electrode 2-6ST and the maximum oscillation frequencyf_(max). With an increase in the t_(ST), the f_(max) also increases, butwhen the t_(ST) is 0.2 μm, the f_(max) is 100 GHz. When the t_(ST) is100 μm, the f_(max) reaches 270 GHz. Thereby, it is possible to providesatisfactory characteristics as compared with the case in theconventional structure where the f_(max) is 81 GHz.

FIG. 12A shows the relationship between the thickness t_(DB) of thelower drain electrode 2-6DB and the maximum oscillation frequencyf_(max). With an increase in the t_(DB), the f_(max) decreases, but whenthe t_(DB) increases from 0.01 μm to 0.2 μm, the decrease in the f_(max)is confined to from 300 GHz to 100 GHz, thus making it possible toprovide satisfactory characteristics as compared with the case in theconventional structure where the f_(max) is 81 GHz.

FIG. 12B shows the relationship between the thickness t_(DT) of theupper drain electrode 2-6DT and the maximum oscillation frequencyf_(max). With an increase in the t_(DT), the f_(max) increases, but whenthe t_(DT) is 0.2 μm, the f_(max) is 100 GHz, and when the t_(DT) is 100μm, the f_(max) reaches 280 GHz. Thereby, it is possible to providesatisfactory characteristics as compared with the case in theconventional structure where the f_(max) is 81 GHz.

Table 1 shows the maximum oscillation frequencies f_(max) of aconventional diamond semiconductor element using Au as a material for asource electrode and a drain electrode, and of a diamond semiconductorelement according to an embodiment of the present invention using anAuPt alloy as a material for the lower source electrode 2-6SB and thelower drain electrode 2-6DB. In this case, the material for the uppersource electrode 2-6ST and the upper drain electrode 2-6DT is Au. Theabove diamond semiconductor element uses Au for the electrode material,but an AuPt alloy having a composition ratio Au:Pt=8:2 is used for theelectrode material of the diamond semiconductor element according to anembodiment of the present invention, resulting in a dramatic increase inf_(max) to 220 GHz.

TABLE 1 Electrode Material and Transistor Characteristics Electrodematerial Maximum oscillation frequency (GHz) Au (gold) conventional art81 Au (gold) Pt (platinum) alloy 220

Table 2 shows the maximum oscillation frequencies f_(max) of aconventional a diamond semiconductor element using Au as the materialfor the source electrode and the drain electrode, and of a diamondsemiconductor element according to an embodiment of the presentinvention using an AuPt alloy, having a composition ratio Au:Pt=8:2, Pt,Pd, Ti, Mo, W as the material for the upper source electrode 2-6ST andthe upper drain electrode 2-6DT. In this case, the material for thelower source electrode 2-6SB and the lower drain electrode 2-6DB is Au.The above diamond semiconductor element uses Au for the electrodematerial, but the electrode materials shown in Table 2 are used for theelectrode material of the diamond semiconductor element according to anembodiment of the present invention, resulting in a dramatic increase inf_(max).

TABLE 2 Electrode Material and Transistor Characteristics Electrodematerial Maximum oscillation frequency (GHz) Au (gold) conventional art81 Au (gold) Pt (platinum) alloy 112 Pt (platinum) 141 Pd (palladium)185 Ti (titanium) 153 Mo (molybdenum) 159 W (tungsten) 143

In this manner, the lower source electrode 2-6SB and the lower drainelectrode 2-6DB may be formed of a material different from that of theupper source electrode 2-6ST and the upper drain electrode 2-6DT.

Next, a process for producing a diamond thin-film according to thepresent invention in order to attain the third object will be describedin detail.

An embodiment of the present invention has the key feature in that adiamond crystal thin-film is fired (annealed) at a sufficienttemperature under high pressure under which a diamond is stable. A trialattempt for improving the crystallinity of a diamond crystal thin-filmhas been made using high-temperature annealing in a vacuum at 1000° C.or more (see non-patent document 5), but deterioration occurs inhigh-temperature vacuum annealing because a diamond is basically stableunder an extremely high pressure of 1.5 Gpa or more. Hence, in anembodiment of the present invention, annealing is performed under highpressures under which a diamond is stable. Thereby, lattice defectsincluded in a crystal, and the like are recovered or removed, thusenabling an enhancement in quality of the diamond crystal thin-film.

In this specification, “(a diamond is) stable or stably” means thecondition in which the diamond maintains its diamond state withoutconversion to graphite. Specifically, “a diamond is stable” means acondition in which the diamond maintains its diamond state withoutconversion to graphite even when being subjected to annealing at hightemperatures and high pressures. This is, for example, in the case ofdiamond of a single crystal, a condition in which a diamondsingle-crystal maintains its single-crystal state without conversion tographite. Therefore, the diamond crystal thin-film is annealed underhigh pressures in such a manner as to allow the diamond to be in itsstable state, thereby preventing or reducing the conversion of thediamond crystal thin-film to graphite.

A temperature (also called annealing temperature) T for performingannealing and a pressure (also called annealing pressure) P forperforming annealing are determined within a region in which a diamondcan be stably annealed. This region satisfies P≧0.71+0.0027 T orP=0.71+0.0027 T, which are shown in FIG. 21, and also is a region ofP≧1.5 GPa. Such a region is the part marked with diagonal lines in FIG.21. The relational expression P≧0.71+0.0027 T is well known to personsskilled in the art. A more preferable temperature T is 550° C.

In FIG. 21, the symbol O and the symbol x indicate the conditions underwhich high-pressure and high-temperature annealing is performed on thediamond crystal thin-film. The symbol O represents the condition underwhich the diamond structure is still stable after the annealing, and thesymbol x represents the condition under which the diamond substrate isconverted to graphite after the annealing.

In an embodiment of the present invention, if the annealing temperatureand the annealing pressure are set within the foregoing region, in otherwords, if the annealing temperature and the annealing pressure, underwhich a diamond is stable, are set, it is possible to reduce thedeterioration of the diamond crystal thin-film. Even when a higherannealing temperature is set, by increasing the annealing pressure, thecondition will fall within the foregoing region, thus making it possibleto set a higher annealing temperature. Thus, a decrease in crystaldefects becomes possible. In consequence, according to the embodiment ofthe present invention, it is possible to achieve an improvement inquality of the diamond crystal thin-film.

A diamond crystal thin-film is formed on a diamond single-crystalsubstrate by use of a microwave plasma CVD apparatus or the like, andthen the diamond single-crystal substrate with the diamond crystalthin-film formed thereon is placed in an ultra-high-pressure andhigh-temperature firing furnace for performing the above-describedannealing.

Two diamond single-crystal substrates with the above-described diamondcrystal thin-films formed thereon are prepared, then the two substratesare superimposed each other such that the surfaces of the respectivediamond crystal thin-films (the face of each diamond crystal thin-filmopposite to the interface between the diamond single-crystal substrateand the diamond crystal thin-film formed thereon) are in contact witheach other, and then annealing may be performed. It should be note thatthe two substrates may be superimposed such that at least parts of thesurfaces of the two diamond crystal thin-films are in contact with eachother. By such superimposition, the area of the surface of the diamondcrystal thin-film exposed to air during the annealing disappears or isreduced, thus making it possible to lessen the effects of oxygen,nitrogen and moisture vapor in the air. Also, the periphery of thesample is surrounded by NaCl and the like during the annealing becausepressure is applied thereto, but the above superimposition makes itpossible to lessen the adhesion of NaCl and the like to the surface ofthe diamond crystal thin-film during the annealing.

In this manner, by superimposing the substrates such that the surfacesof the two diamond crystal thin-films face inward toward each other,each of the two diamond crystal thin-films can function as a protectivemember for the other diamond crystal thin-film. In other words, at leasta part of the surface of one of the diamond crystal thin-films is incontact with at least a part of the surface of the other diamond crystalthin-film, whereby one diamond crystal thin-film overlies the contactarea of the other. By performing the annealing on the substrates thussuperimposed, while the two diamond crystal substrates mutually protecttheir surfaces, it is possible to perform the annealing simultaneouslyon the two substrates approximately in the same position, oralternatively it is possible to perform the annealing in a smaller spacethan the space required for annealing the two substrates individually.In consequence, because the space for performing the annealing can bereduced, it is possible to increase the number of substrates on whichthe annealing is performed at a time, thus making it possible to moreefficiently produce high quality diamond crystal thin-films.

Further, a protective member such as an insulating thin-film, a metalthin-film or an alloy, may be formed on at least a part of the surfaceof the diamond crystal thin-film and then the annealing may beperformed. Because at least a part of the surface of the diamond crystalthin-film is overlaid with the protective member in this manner and thenthe annealing is performed, it is possible to lessen the effects ofoxygen, nitrogen and moisture vapor in the air during the annealing, asin the above-described case of superimposing the substrates each other.Also, it is possible to lessen the adhesion of NaCl and the like to thesurface of the diamond crystal thin-film during the annealing.

It should be noted that, in this specification, the “protective member”is a member for lessening the effects of each component in the air, andof the materials such as NaCl and the like used for the application ofpressure, on the surface of the diamond crystal thin-film. Theprotective member exerts its function by overlying or by being formed onat least a part of the surface of the diamond crystal thin-film.Specifically, the existence of the protective member on the diamondcrystal thin-film makes it possible to prevent or lessen the contact andadhesion of the components of the air, such as oxygen, nitrogen andmoisture vapor, and of materials such as NaCl used for the applicationof pressure, to the surface of the diamond crystal thin-film.

It should be noted that, in an embodiment of the present invention, thesubstrate on which the diamond crystal thin-film is formed is notlimited to the diamond single-crystal substrate. For example, anothersubstrate such as a diamond polycrystal substrate or a siliconesubstrate may be used.

Further, the diamond crystal thin-film formed on the substrate may be adiamond single-crystal thin-film or a diamond polycrystal thin-film.

A process for producing a diamond thin-film according to an embodimentof the present invention will be described below in detail on the basisof embodiment examples, but it goes without saying that the presentinvention is not limited to the embodiment examples, and many changesand variations can be made without departing from the scope of thepresent invention. For example, the diamond single-crystal thin-film mayhave any thickness, and is not limited to the values described in thefollowing embodiment examples.

Embodiment Example 1

A process for producing a diamond thin-film according to embodimentexample 1 of the present invention will be described using FIGS. 15A to15D. A diamond single-crystal substrate 3-11 having surface orientation(100) is prepared (FIG. 15A). Then, a microwave plasma CVD apparatus isused to deposit a diamond single-crystal thin-film 3-12 of about 1-5 μmthickness onto the diamond single-crystal substrate 3-11 at a substratetemperature of 700° C. by use of methane as a reaction gas (FIG. 15B).The diamond single-crystal thin-film formed in such a manner maypossibly include defects and/or impurities. In the embodiment example,the microwave plasma CVD apparatus is used, but any growing techniquemay be used as long as it can form a diamond thin-film.

After that, the diamond single-crystal substrate 3-11 having the diamondsingle-crystal thin-film 3-12 formed thereon is placed in anultra-high-pressure and high-temperature firing furnace to anneal thediamond single-crystal thin-film 3-12 under the conditions of 1200° C.and 6 GPa (FIG. 15C). A high-quality diamond crystal thin-film withdecreased defects and impurities can be obtained by the annealing (FIG.15D).

It should be noted that, in a preliminary experiment, a diamondsingle-crystal substrate having a diamond crystal thin-film formedthereon is annealed under conditions of various temperatures andpressures as shown in FIG. 21, in which the conditions of 1200° C. and 6GPa is positioned in the region where a diamond can be stably annealed.

The holes in the diamond crystal thin-film are measured before and afterthe high-pressure and high-temperature annealing for a comparison ofcharacteristics. In this connection, the mobility of the semiconductorwhich is obtained by the hole measurement is closely associated with thecrystallinity, and the better the crystallinity, the more the mobilityincreases. In the embodiment example, some samples are produced underthe same condition in order to reduce the variation between the samples,and the hole measurement is carried out to obtain an average value ofthe mobility (the average mobility).

As seen from Table 3, the average mobility of the yet-to-be-annealedsample (FIG. 15B), or the sample produced by a conventional process, atroom temperature is about 800 cm²/Vs. On the other hand, the averagemobility of the sample subjected to the high-pressure andhigh-temperature annealing (FIG. 15D) is 1000 cm²/Vs, resulting in anincrease in the average mobility.

Further, the diamond crystal thin-films before and after thehigh-temperature and high-pressure annealing process are used to producea field-effect transistor (FET), and current-voltage (Ids-Vds)measurement is conducted for a comparison of characteristics (FIG. 23).For this FET, gold is used as source and drain electrodes and aluminumis used as a gate electrode. The gate length is 5 μm and the gate widthis 100 μm. The gate voltage is 0-3.5V and is measured in 0.5V steps.

As seen from FIG. 23, current leakage appearing before annealing isalmost eliminated after annealing, resulting in an improvement incharacteristics.

It is seen from these results that, by performing the high-pressure andhigh-temperature annealing on a diamond crystal thin-film formed on asubstrate, the defects in the diamond crystal thin-film decrease and thediamond crystal thin-film is increased in quality. At this stage,because the annealing temperature and the annealing pressure are setsuch that a diamond is stable, it is possible to suppress deteriorationof the diamond crystal thin-film. In addition, the annealing temperaturecan be increased as long as it falls within the region where the diamondcan be annealed, thus achieving a further reduction in defects.

TABLE 3 Average hole mobility at room temperature (cm²/Vs) Conventionalart 800 Embodiment Example 1 1000 Embodiment Example 2 1300 EmbodimentExample 3 1300 Embodiment Example 4 1300 Embodiment Example 5 1200Embodiment Example 6 1500

Embodiment Example 2

A process for producing a diamond thin-film according to embodimentexample 2 of the present invention will be described using FIGS. 16A to16D. A diamond single-crystal substrate 3-21 having surface orientation(100) is prepared (FIG. 16A). Then, a microwave plasma CVD apparatus isused to deposit a diamond single-crystal thin-film 3-22 of about 1 μm toabout 5 μm thickness onto the diamond single-crystal substrate 3-21 at asubstrate temperature of 700° C. by use of methane as a reaction gas(FIG. 16B). The diamond single-crystal thin-film thus formed maypossibly include defects and/or impurities. In the embodiment example,two diamond single-crystal substrates 3-21 having the diamondsingle-crystal thin-films 3-22 formed as described above are prepared.

After that, the two substrates are superimposed each other such that, asillustrated in FIG. 16C, the surfaces of the respective diamondsingle-crystal thin-films 3-22 face inwardly, that is, such that thesurfaces of the two diamond single-crystal thin-films 3-22 make contactwith each other, and then the superimposed substrates are placed in anultra-high-pressure and high-temperature firing furnace and thenannealed under the conditions of 1200° C. and 6 GPa.

The above-described superimposition may be carried out by hand oralternatively by use of positioning means having means for picking upthe substrate and putting it in position, and driving means for drivingthe means. In the case of using such positioning means, the drivingmeans such as a motor is operated to pick up one of the diamondsingle-crystal substrate 3-21 and place it onto the other diamondsingle-crystal substrate 3-21 such that the surfaces of the two diamondsingle-crystal thin-films 3-22 are aligned in contact with each other.

In this manner, in the embodiment example, any means may be used as longas the substrates can be superimposed in such a manner as to makecontact between the surfaces of the two diamond single-crystalthin-films 3-22.

Upon the termination of the annealing, the two superimposed substratesare separated from each other, and high-quality diamond crystalthin-films reduced in defects and impurities through the above-describedannealing are obtained (FIG. 16D).

Hole measurement is conducted on the diamond crystal thin-film beforeand after the high-pressure and high-temperature annealing for acomparison of characteristics. As seen from Table 3, the averagemobility of the yet-to-be-annealed sample (FIG. 16B), or the sampleproduced by a conventional process, at room temperature is about 800cm²/Vs. On the other hand, the average mobility of the sample subjectedto the high-pressure and high-temperature annealing (FIG. 16D) is 1300cm²/Vs, resulting in an increase in the average mobility. It isunderstood from these results that the defects in the diamond crystalthin-film are reduced by the high-pressure and high-temperatureannealing, thus increasing the quality of the diamond thin-film. Inaddition, in the embodiment example, the surfaces of the diamond crystalthin-film are in contact with each other during the annealing process,which makes it possible to lessen the effects of oxygen, nitrogen,moisture vapor and the like in the air on the diamond crystal thin-film.Also, the above superimposition makes it possible to lessen the adhesionof NaCl, which is disposed around the substrate for the application ofpressure, to the surface of the diamond crystal thin-film during theannealing.

It should be noted that the embodiment example has described thesuperimposition of the surfaces of the two diamond single-crystalthin-films 3-22 each other with their whole surfaces making contact witheach other, but is not limited to this. What is important in theembodiment example is to lessen the effects of each component of theair, NaCl used for the application of pressure, and the like on thesurface of the diamond single-crystal thin-film 3-22 during theannealing, and for this end, a matter of importance is that no part ofthe surface is exposed during the annealing. Thus, in an embodimentexample of the invention, if the two substrates are superimposed suchthat at least parts of the surfaces of the two diamond crystalthin-films make at least partial contact with each other, the foregoingeffects are lessened.

In this manner, in the embodiment example, what is required is to reducethe exposed area of the surface of the diamond crystal thin-film inorder to lessen the effects of each component of the air, NaCl used forthe application of pressure, and the like on the surface of the diamondcrystal thin-film during the annealing. Hence, the procedure for this isnot limited to the superimposition of the two diamond crystal thin-filmssuch that their surfaces face inward. For example, annealing may beperformed by placing a diamond crystal substrate or oxides, nitrides,metal, alloy or the like, described in embodiment examples 3 and 4, as aprotective member on at least a part of the diamond crystal thin-film,so as to overlie at least the part with the protective member.

Embodiment Example 3

A process for producing a diamond thin-film according to embodimentexample 3 of the present invention will be described using FIGS. 17A to17F. A diamond single-crystal substrate 3-31 having surface orientation(100) is prepared (FIG. 17A). Then, a microwave plasma CVD apparatus isused to deposit a diamond single-crystal thin-film 3-32 of about 1 μm toabout 5 μm thickness onto the diamond single-crystal substrate 3-31 at asubstrate temperature 700° C. by use of methane as a reaction gas (FIG.17B). The diamond single-crystal thin-film thus formed may possiblyinclude defects and/or impurities.

After that, a protective film 3-33 as the protective member is formed onthe diamond single-crystal thin-film 3-32 as illustrated in FIG. 17C.The protective film 3-33 may be formed of various metallic compoundshaving a film-thickness of about 0.5 μm. Examples of the material forthe protective film 3-33 include, but are not limited to, siliconmonoxide (SiO_(x)), silicon nitride (SiN_(x)), aluminum oxide (AlO_(x))and the like. These silicon monoxide, silicon nitride, aluminum oxidemay be formed by an ECR spattering technique.

Next, the diamond single-crystal substrate 3-31 with the protective film3-33 and the diamond single-crystal thin-film 3-32 formed thereon isplaced in an ultra-high-pressure and high-temperature firing furnace, toanneal the diamond single-crystal thin-film 3-32 under the conditions of1200° C. and 6 GPa (FIG. 17D). By the annealing, a high-quality diamondcrystal thin-film with reduced defects and impurities is obtained (FIG.17E). Then, etching is performed for removing the protective film 3-33(FIG. 17F).

Hole measurement is conducted on the diamond crystal thin-film beforeand after the high-pressure and high-temperature annealing for acomparison of the characteristics of the samples. As seen from Table 3,the average mobility of the yet-to-be-annealed sample (FIG. 17B), or ofthe sample produced by a conventional process, at room temperature isabout 800 cm²/Vs. On the other hand, the average mobility of the samplesubjected to the high-pressure and high-temperature annealing (FIG. 17F)is 1300 cm²/Vs regardless of the type of protective film, which ishigher than that produced by the conventional process. It can be seenfrom these results that the defects in the diamond crystal thin-film arereduced by the high-pressure and high-temperature annealing, thusincreasing the quality of the diamond crystal thin-film. In addition, inthe embodiment example, because the protective film is formed on thediamond crystal thin-film, it is possible during the annealing to lessenthe effects of nitrogen, oxygen, moisture vapor and the like in the airon the surface of the diamond crystal thin-film. Also, theaforementioned protective film makes it possible to lessen the adhesionof NaCl, which is disposed around the substrate for the application ofpressure, to the surface of the diamond crystal thin-film during theannealing.

It should be noted that the embodiment example has described theformation of the protective film 3-33 on the full surface of the diamondsingle-crystal thin-film 3-32, but is not limited to this. What isimportant in the embodiment example is to lessen the effects of eachcomponent of the air, NaCl used for the application of pressure, and thelike on the surface of the diamond single-crystal thin-film 3-32 duringthe annealing, and for this end, a matter of importance is that no partof the surface is exposed during the annealing. Thus, in an embodimentexample of the invention, if a protective film is formed on at least apart of the diamond single-crystal thin-film, the foregoing effects canbe lessened.

Embodiment Example 4

A process for producing a diamond thin-film according to embodimentexample 4 of the present invention will be described using FIGS. 18A to18F. A diamond single-crystal substrate 3-41 having surface orientation(100) is prepared (FIG. 18A). Then, a microwave plasma CVD apparatus isused to deposit a diamond single-crystal thin-film 3-42 of about 1 μm toabout 5 μm thickness onto the diamond single-crystal substrate 3-41 at asubstrate temperature 700° C. by use of methane as a reaction gas (FIG.18B). The diamond single-crystal thin-film thus formed may possiblyinclude defects and/or impurities.

After that, a protective film 3-43 as a protective member is formed onthe diamond single-crystal thin-film 3-42 as illustrated in FIG. 18C. Asthe protective film 3-43, various metallic films or alloy films having afilm-thickness of about 0.5 μm may be used. Examples of the material forthe protective film 3-43 include, but are not limited to, platinum (Pt),titanium (Ti), tungsten (W), palladium (Pd), molybdenum (Mo), atitanium-aluminum alloy (Ti65%-Al35%) and the like. These platinum,titanium, tungsten, palladium, molybdenum, titanium-aluminum alloy andthe like may be formed by a vacuum evaporation technique.

A titanium-aluminum alloy is used as an alloy in the embodiment example,but the alloy may include a metal of at least one of Pt, Ti, W, Pd, andMo.

Next, the diamond single-crystal substrate 3-41 with the protective film3-43 and the diamond single-crystal thin-film 3-42 formed thereon isplaced in an ultra-high-pressure high temperature firing furnace, toanneal the above diamond single-crystal thin-film 3-42 under theconditions of 1200° C. and 6 GPa (FIG. 18D). By the annealing, ahigh-quality diamond crystal thin-film with reduced defects andimpurities is obtained (FIG. 18E). Then, etching is performed forremoving the protective film 3-43 (FIG. 18F).

Hole measurement is conducted on the diamond crystal thin-film beforeand after the high-pressure and high-temperature annealing for acomparison of the characteristics of the samples. As seen from Table 3,the average mobility of the yet-to-be-annealed sample (FIG. 18B), or ofthe sample produced by a conventional process, at room temperature isabout 800 cm²/Vs. On the other hand, the average mobility of the samplesubjected to the high-pressure and high-temperature annealing (FIG. 18F)is 1300 cm²/Vs regardless of the type of protective film, which ishigher than that produced by the conventional process. It is understoodfrom these results that the defects in the diamond crystal thin-film arereduced by the high-pressure and high-temperature annealing, thusincreasing the quality of the diamond crystal thin-film. In addition, inthe embodiment example, because the protective film is formed on thediamond crystal thin-film, it is possible during the annealing to lessenthe effects of nitrogen, oxygen, moisture vapor and the like in the airon the surface of the diamond crystal thin-film. Also, theaforementioned protective film makes it possible to lessen the adhesionof NaCl, which is disposed around the substrate for the application ofpressure, to the surface of the diamond crystal thin-film during theannealing.

Embodiment Example 5

A process for producing a diamond thin-film according to embodimentexample 5 of the present invention will be described using FIGS. 19A to19D. A diamond single-crystal substrate 3-51 having surface orientation(111) is prepared (FIG. 19A). Then, a microwave plasma CVD apparatus isused to deposit a diamond single-crystal thin-film 3-52, having surfaceorientation (111), of about 1 μm to about 5 μm thickness onto thediamond single-crystal substrate 3-51 at a substrate temperature 700° C.by use of methane as a reaction gas (FIG. 19B). The diamondsingle-crystal thin-film thus formed may possibly include defects and/orimpurities.

After that, the diamond single-crystal substrate 3-51 with the diamondsingle-crystal thin-film 3-52 formed thereon is placed in anultra-high-pressure and high-temperature firing furnace, to anneal theabove diamond single-crystal thin-film 3-52 under the conditions of1200° C. and 6 GPa (FIG. 19C). By the annealing, a high-quality diamondcrystal thin-film with reduced defects and impurities is obtained (FIG.19D).

Hole measurement is conducted on the diamond crystal thin-film beforeand after the high-pressure and high-temperature annealing for acomparison of the characteristics of the samples. As seen from Table 3,the average mobility of the yet-to-be-annealed sample (FIG. 19B), or ofthe sample produced by a conventional process, at room temperature isabout 800 cm²/Vs. On the other hand, the average mobility of the samplesubjected to the high-pressure and high-temperature annealing (FIG. 19D)is 1200 cm²/Vs, resulting in an increase in the average mobility. It isunderstood from these results that the defects in the diamond crystalthin-film are reduced by the high-pressure and high-temperatureannealing, thus increasing the quality of the diamond crystal thin-film.

Embodiment Example 6

A process for producing a diamond thin-film according to embodimentexample 6 of the present invention will be described using FIGS. 20A to20D. A diamond single-crystal substrate 3-61 having surface orientation(100) is prepared (FIG. 20A). Then, a microwave plasma CVD apparatus isused to deposit a diamond single-crystal thin-film 3-62 of about 1 μm toabout 5 μm thickness onto the diamond single-crystal substrate 3-61 at asubstrate temperature 650° C. to 700° C. by use of methane as a reactiongas (gas concentration 0.5%) (FIG. 20B). In the embodiment example,because a substrate temperature (a temperature at which the substrate isheated when a diamond crystal thin-film is formed thereon), a flow rateof methane, and the like are controlled while the diamond crystalthin-film is growing, it is possible to obtain a diamond crystalthin-film having surface smoothness of 30 nm or less in a 1 μm² regionin terms of average square roughness.

The conditions of controlling the substrate temperature and the flowrate of methane will be described below. That is, the substratetemperature is controlled to be a temperature of no less than 650° andno more than 700° C. Specifically, the substrate temperature iscontrolled to be no less than a growing temperature when a diamondcrystal thin-film is formed and no more than a temperature of 700° C.The flow rate of methane is controlled such that the flow rate ofmethane divided by the flow rate of hydrogen, i.e., the ratio of theflow rate of methane to the flow rate of hydrogen (the ratio of the flowrate (sccm)), is greater than 0% and no more than 0.5%. Since thediamond crystal thin-film does not grow when the reaction gas comprisesonly hydrogen (the above ratio is 0%), the above ratio is required to beset greater than 0%. However, when the ratio exceeds 0% and is less than0.1%, the growing speed is slower as compared with the case when theratio is no less than 0.1% and no more than 0.5%. As a result, the ratiois preferably set no less than 0.1% and no more than 0.5%.

That is, in the embodiment example, by controlling at least one ofcontrols of the substrate temperature and of the flow rate of methane,the surface evenness of the diamond crystal thin-film is able to beenhanced.

The diamond single-crystal thin-film thus formed may possibly includedefects and/or impurities. In the embodiment example, two diamond singlecrystal substrates 3-61 with the diamond single-crystal thin-films 3-62produced as described above are prepared.

Then, the substrates are superimposed such that the two diamondsingle-crystal thin-film 3-62 face inward as illustrated in FIG. 20C,then the superimposed substrates are placed in an ultra-high-pressureand high-temperature firing furnace, and then annealing is carried outunder the conditions of 1200° C. and 6 GPa. Upon the completion of theannealing, the two superimposed substrates are separated from eachother, thus obtaining high-quality diamond crystal thin-films withreduced defects and impurities through the above-described annealing(FIG. 20D).

Hole measurement is conducted on the diamond crystal thin-film beforeand after the high-pressure and high-temperature annealing for acomparison of characteristics. As seen from Table 3, the averagemobility of the yet-to-be-annealed sample (FIG. 20B), or the sampleproduced by a conventional process, at room temperature is about 800cm²/Vs. On the other hand, the average mobility of the sample subjectedto the high-pressure and high-temperature annealing (FIG. 20D) is 1500cm²/Vs, resulting in an increase in the average mobility. It isunderstood from these results that the defects in the diamond crystalthin-film are reduced by the high-pressure and high-temperatureannealing, thus increasing the quality of the diamond thin-film.

Next, a p-type diamond semiconductor and a process for producing thep-type diamond semiconductor according to the present invention in orderto attain the fourth object will be described in detail.

In the present invention, for the purpose of simultaneous improvement inthe hole concentration and the dopant atom concentration in the p-typediamond semiconductor, aluminum (Al), beryllium (Be), calcium (Ca),cadmium (Cd), gallium (Ga), indium (In), magnesium (Mg) or zinc (Zn),which have activation energy lower than that of B, is used as a p-typedopant element for diamond. As a result, the use of the p-type diamondin a part of the structure makes it possible to realize a semiconductorelement for a MES-type and a MIS-type field-effect transistor (FET), aPNP-type and a NPN-type bipolar transistor, a semiconductor laser and alight-emitting diode, which functions even at 300K.

Embodiment 4

In a microwave plasma chemical vapor deposition technique, by using as afeedstock a gas mixture of a total flow rate of 300 ccm comprising areaction gas including a methane gas (CH₄) at a flow ratio of 1%, adopant gas and H₂ as the remainder, a diamond semiconductor film of thepresent invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. The pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW. Here, as the dopant gas, eithertrimethylaluminum ((CH₃)₃Al: TMAl) or triethylaluminum ((C₂H₅)₃Al:TEAl), which are organometallic materials including Al, is used.Alternatively, instead of the dopant gas, solid-state Al may beintroduced into plasma and the vaporized Al may be used as a dopant gas.

Hole measurement is made on the diamond semiconductor films thusobtained to evaluate the hole coefficient, whereby it can be confirmedthat these diamond semiconductor films are p-type semiconductors.

FIG. 24 shows the relationship between the concentration of dopant atomsin a source gas and the concentration of dopant atoms in a p-typediamond semiconductor, according to the embodiment of the presentinvention. By use of SIMS (Secondary Ion Mass Spectrometer) measurement,the concentration of Al atoms in the diamond semiconductor film ismeasured, and when the ratio of the number of Al atoms to the number ofcarbon (C) atoms in the source gas (Al/C)x(ppm) is represented by thehorizontal axis and the concentration of AL atoms in the semiconductorfilm y(cm⁻³) is represented by the vertical axis, the relationship is,as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

FIG. 25 shows the temperature dependence of a hole concentration foreach Al atom concentration in a p-type diamond semiconductor accordingto embodiment 4 of the present invention. In FIG. 25 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Al atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at 300K whichis a practical level, an Al atom concentration of 2.0×10¹⁷ cm⁻³ or moreis required. In the case of the Al atom concentration exceeding 1.0×10²¹cm⁻³, it is known that deterioration in the quality of the diamondcrystal occurs. Accordingly, the dopant atom concentration in theAl-doped p-type diamond semiconductor element at 300K which is apractical level is required to be no less than 2.0×10¹⁷ cm⁻³ and no morethan 1.0×10²¹ cm⁻³.

It is seen from this that, using equation (1), the ratio (Al/C) of thenumber of Al atoms to the number of C atoms in the source gas isrequired to fall within a range of from no less than 2 ppm to no morethan 10⁴ ppm.

Using FIG. 25 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for eachconcentration of the dopant atoms, a dopant atom concentration enablingthe hole concentration in the p-type diamond semiconductor to reach1.0×10¹⁵ cm⁻³ at each temperature can be found. For example, 2.6×10¹⁶cm⁻³ is derived from FIG. 25 as an Al atom concentration enabling thehole concentration in the p-type diamond semiconductor to reach 1.0×10¹⁵cm⁻³ at 500K. It is seen from equation (1) that the ratio (Al/C) of thenumber of Al atoms to the number of C atoms in the source gas in thiscase is required to fall within the range of from no less than 26 ppm tonot more than 10⁴ ppm.

In the range of no more than 1.0×10²¹ cm⁻³, the higher the Al atomconcentration in the p-type diamond semiconductor film, the higher thehole concentration that is obtained, resulting in, needless to say, adiamond semiconductor further suited to practical use.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is no less than 1.0×10¹⁵ cm⁻³ and the dopant atomconcentration is no more than 1.0×10²¹ cm⁻³. For this reason, the p-typediamond semiconductor according to embodiment 4 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Al atom concentration 1.0×10²¹ cm⁻³. For example, embodiment4 can function as the p-type semiconductor even in any temperature otherthan 300K. The region in which the function as a p-type semiconductor isachieved in embodiment 4 is considerably wider than that in the case ofa conventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, Table 4 shows the mobility, at 300K, of holes in each of thep-type diamond semiconductor films obtained when trimethylaluminum((CH₃)₃Al: TMAl), triethylaluminum ((C₂H₅)₃Al: TEAl) or aluminiumchloride (AlCl₃) of the present invention is used as a dopant gas.

TABLE 4 (All in Al atom concentration of 1.0 × 10¹⁹ cm⁻³) Al dopant Holemobility at room material temperature (cm²/Vs) TMAl 1500 TEAl 1400 AlCl₃200

As shown in Table 4, the hole mobility at room temperature in the caseof using trimethylaluminum ((CH₃)₃Al: TMAl) or triethylaluminum((C₂H₅)₃Al: TEAl) is about 7 times higher than that in the case of usingaluminium chloride (AlCl₃), resulting in the diamond semiconductorhaving notably outstanding characteristics.

Embodiment 5

Be may be used as dopant and an ion implantation technique may be usedto implant the Be dopant into a diamond single-crystal under theconditions of an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻²to produce Be-impurity-doped diamond. As an alternative to the ionimplantation technique, the Be-impurity-doped diamond may be produced bya microwave plasma chemical vapor deposition technique in whichsolid-state Be may be introduced into plasma and the vaporized Be isused as a dopant gas. In the microwave plasma chemical vapor depositiontechnique, by using as a feedstock a reaction gas of a total flow rateof 300 ccm comprising a methane gas (CH₄) at a flow ratio of 1%,solid-state Be, and H² as the remainder, a diamond semiconductor film ofthe present invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. Here, the pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW.

Then, annealing is performed on the Be-impurity-doped diamond thusobtained. Hole measurement is conducted on the Be-doped diamondsemiconductor film of the present invention thus obtained to evaluatethe hole coefficient, whereby it can be confirmed that this diamondsemiconductor film is a p-type semiconductor.

By use of SIMS measurement, the Be atom concentration y in the diamondsemiconductor film is measured, and when the ratio of the number of Beatoms to the number of C atoms in the source gas (Be/C)x(ppm) isrepresented by the horizontal axis and the concentration of Be atoms inthe semiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

Through the hole measurement, the temperature dependence of the holeconcentration in a sample of each of the Be dopant atom concentrationsfrom 1.0×10¹⁶ cm⁻³ to 1.0×10²¹ cm⁻³, is measured. From this result, FIG.26 shows the temperature dependence of a hole concentration for each Beatom concentration in the p-type diamond semiconductor according toembodiment 5 of the present invention. In FIG. 26 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Be atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Be atom concentration of no less than7.0×10¹⁶ cm⁻³ is required. It is known that a deterioration in thequality of the diamond crystal occurs in the Be atom concentrationexceeding 1.0×10²¹ cm⁻³. Accordingly, the dopant atom concentration inthe Be-doped p-type diamond semiconductor element at about 300K in thepractical level is required to be no less than 7.0×10¹⁶ cm⁻³ and no morethan 1.0×10²¹ cm⁻³.

It is seen from this that, using equation (1), a ratio (Be/C) of thenumber of Be atoms to the number of C atoms in the source gas isrequired to fall within a range from no less than 0.7 ppm to no morethan 10⁴ ppm.

Using FIG. 26 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for eachconcentration of the dopant atoms, a dopant atom concentration enablingthe hole concentration in the p-type diamond semiconductor to reach1.0×10¹⁵ cm⁻³ at each temperature can be found. For example, 1.3×10¹⁶cm⁻³ is derived from FIG. 26 as a Be atom concentration enabling thehole concentration in the p-type diamond semiconductor to reach 1.0×10¹⁵cm⁻³ at 500K.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the Be atomconcentration, the higher the hole concentration, resulting in, needlessto say, a diamond semiconductor further suited to practical use.

Also, in the P-type diamond semiconductor according to the embodiment itis possible to obtain a room-temperature hole concentration about 25000times that in a conventional p-type diamond semiconductor doped with B,with the same dopant atom concentration.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is no less than 1.0×10¹⁵ cm⁻³ and the dopant atomconcentration is no more than 1.0×10²¹ cm⁻³. For this reason, the p-typediamond semiconductor according to embodiment 5 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Be atom concentration 1.0×10²¹ cm⁻³. For example, embodiment5 can function as a p-type semiconductor even in any temperature otherthan 300K. The region enabling functioning as a p-type semiconductor inembodiment 5 is considerably wider than that in the case of aconventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Embodiment 6

Ca may be used as dopant and an ion implantation technique may be usedto implant the Ca dopant into a diamond single-crystal under theconditions of an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻²to produce Ca-impurity-doped diamond. As an alternative to the ionimplantation technique, the Ca-impurity-doped diamond may be produced bya microwave plasma chemical vapor deposition technique. In the microwaveplasma chemical vapor deposition technique, by using as a feedstock agas mixture of a total flow rate of 300 ccm comprising a reaction gasincluding a methane gas (CH₄) at a flow ratio of 1%, a dopant gas and H²as the remainder, a diamond semiconductor film of the present inventionis grown to 1.0 μm thickness on a diamond single-crystal of (001)surface orientation. The pressure in the reaction tube is 50 Torr andthe microwave source has a frequency of 2.45 GHz and a power of 1.3 kW.Here, calcium chloride CaCl₂ is used as the dopant gas. Alternatively,instead of the dopant gas, solid-state Ca may be introduced into plasmaand the vaporized Ca may be used as a dopant gas.

By use of SIMS measurement, the Ca atom concentration y in the diamondsemiconductor film is measured, and when the ratio of the number of Caatoms to the number of C atoms in the source gas (Ca/C)x(ppm) isrepresented by the horizontal axis and the concentration of Ca atoms inthe semiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

Through the hole measurement, the temperature dependence of the holeconcentration in a sample of each of the Ca dopant atom concentrationsfrom 1.0×10¹⁶ cm⁻³ to 1.0×10²¹ cm⁻³, is measured. From this result, FIG.27 shows the temperature dependence of a hole concentration for each Caatom concentration in the p-type diamond semiconductor according toembodiment 6 of the present invention. In FIG. 27 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Ca atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Ca atom concentration of no less than3.0×10¹⁷ cm⁻³ is required. The deterioration in the quality of thediamond crystal occurs in Ca atom concentration exceeding 1.0×10²¹ cm⁻³.Accordingly, the dopant atom concentration in the Ca-doped p-typediamond semiconductor element at about 300K which is a practical levelis required to be no less than 3.0×10¹⁷ cm⁻³ and no more than 1.0×10²¹cm⁻³.

It is seen from this that, using equation (1), a ratio (Ca/C) of thenumber of Ca atoms to the number of C atoms in the source gas isrequired to fall within a range from no less than 3.0 ppm to no morethan 10⁴ ppm.

Using FIG. 27 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for each dopantatom concentration, a dopant atom concentration enabling the holeconcentration in the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³at each temperature can be found. For example, 3.2×10¹⁶ cm⁻³ is derivedfrom FIG. 27 as a Ca atom concentration enabling the hole concentrationin the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³ at 500K. Itis seen from equation (1) that the ratio (Ca/C) of the number of Caatoms to the number of C atoms in the source gas in this case isrequired to fall within the range from 0.32 ppm or more to 10⁴ ppm orless.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the Ca atomconcentration, the higher the hole concentration, resulting in, needlessto say, a diamond semiconductor further suited to practical use.

Also, in the P-type diamond semiconductor according the embodiment it ispossible to obtain a room-temperature hole concentration about 5.7×10³times that in a conventional p-type diamond semiconductor doped with B,with the same dopant atom concentration.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is no less than 1.0×10¹⁵ cm⁻³ and the dopant atomconcentration is no more than 1.0×10²¹ cm⁻³. For this reason, the p-typediamond semiconductor according to embodiment 6 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Ca atom concentration 1.0×10²¹ cm⁻³. For example, embodiment6 can function as a p-type semiconductor even in any temperature otherthan 300K. The region enabling functioning as a p-type semiconductor inembodiment 6 is considerably wider than that in the case of aconventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, the mobility, at room temperature, of holes in the p-type diamondsemiconductor film provided by the embodiment is 200 cm²/Vs under theconditions of a Ca atom concentration of 1.0×10¹⁹ cm⁻³.

Embodiment 7

In a microwave plasma chemical vapor deposition technique, by using as afeedstock a gas mixture of a total flow rate of 300 ccm comprising areaction gas including a methane gas (CH₄) at a flow ratio of 1%, adopant gas and H² as the remainder, a diamond semiconductor film of thepresent invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. The pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW. Here, as the dopant gas, eitherdimethylcadmium ((CH₃)₂Cd: DMCd) or diethylcadmium ((C₂H₅)₂Cd: DECd),which is an organometallic material including Cd, is used.

Hole measurement is conducted on the obtained diamond semiconductorfilms to evaluate the hole coefficient, whereby it can be confirmed thatthese diamond semiconductor films are p-type semiconductors.

Also, the Cd atom concentration in the diamond semiconductor film ismeasured by the SIMS measurement, and when the ratio of the number of Cdatoms to the number of C atoms in the source gas (Cd/C)x(ppm) isrepresented by the horizontal axis and the Cd atom concentration in thesemiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

FIG. 28 shows the temperature dependence of a hole concentration foreach Cd atom concentration in a p-type diamond semiconductor accordingto embodiment 7 of the present invention. In FIG. 28 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Cd atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Cd atom concentration of no less than2.0×10¹⁶ cm⁻³ is required. It is known that deterioration in the qualityof the diamond crystal occurs in the Cd atom concentration exceeding1.0×10²¹ cm⁻³. Accordingly, the dopant atom concentration in theCd-doped p-type diamond semiconductor element at a practical level isrequired to be no less than 2.0×10¹⁶ cm⁻³ and no more than 1.0×10²¹cm⁻³.

It is seen from this that, using equation (1), the ratio (Cd/C) of thenumber of Cd atoms to the number of C atoms in the source gas isrequired to fall within a range from no less than 0.2 ppm to no morethan 10⁴ ppm.

Using FIG. 28 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for each dopantatom concentration, the dopant atom concentration enabling the holeconcentration in the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³at each temperature can be found. For example, 6.4×10¹⁵ cm⁻³ is derivedfrom FIG. 28 as a Cd atom concentration enabling the hole concentrationin the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³ at 500K. Itis seen from equation (1) that the ratio (Cd/C) of the number of Cdatoms to the number of C atoms in the source gas in this case isrequired to fall within the range of from no less than 0.064 ppm to nomore than 10⁴ ppm.

In the range of no more than 1.0×10²¹ cm⁻³, the higher the Cd atomconcentration in the p-type diamond semiconductor film, the higher thehole concentration, resulting in, needless to say, a diamondsemiconductor further suited to practical use.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is no less than 1.0×10¹⁵ cm⁻³ and the dopant atomconcentration is no more than 1.0×10²¹ cm⁻³. For this reason, the p-typediamond semiconductor according to embodiment 7 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Cd atom concentration 1.0×10²¹ cm⁻³. For example, embodiment7 can function as the p-type semiconductor even in any temperature otherthan 300K. The region enabling functioning as a p-type semiconductor inembodiment 7 is considerably wider than that in the case of aconventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, Table 5 shows the comparison result of the mobility, at 300K, ofholes of the p-type diamond semiconductor films which respectively areobtained when dimethylcadmium ((CH₃)₂Cd: DMCd), diethylcadmium((C₂H₅)₂Cd: DECd) or cadmium chloride (CdCl₂) of the present inventionis used as a dopant gas.

TABLE 5 (All in Cd atom concentration of 1.0 × 10¹⁹ cm⁻³) Cd dopantmaterial Hole mobility at room temperature (cm²/Vs) DMCd 1350 DECd 1250CdCl₂ 150

As shown in Table 5, the room-temperature hole mobility in the case ofusing dimethylcadmium ((CH₃)₂Cd: DMCd) and diethylcadmium ((C₂H₅)₂Cd:DECd) is about 8 times or more higher than that in the case of usingcadmium chloride (CdCl₂), resulting in the diamond semiconductor havingnotably outstanding characteristics.

Embodiment 8

In a microwave plasma chemical vapor deposition technique, by using as afeedstock a gas mixture of a total flow rate of 300 ccm comprising areaction gas including a methane gas (CH₄) at a flow ratio of 1%, adopant gas and H² as the remainder, a diamond semiconductor film of thepresent invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. The pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW. Here, as the dopant gas, eithertrimethylgallium ((CH₃)₃Ga: TMGa) or triethylgallium ((C₂H₅)₃Ga: TEGa),which is an organometallic material including Ga, is used.

Hole measurement is conducted on the obtained diamond semiconductorfilms to evaluate the hole coefficient, whereby it can be confirmed thatthese diamond semiconductor films are p-type semiconductors.

Also, the Ga atom concentration in the diamond semiconductor film ismeasured by the SIMS measurement, and when the ratio of the number of Gaatoms to the number of C atoms in the source gas (Ga/C)x(ppm) isrepresented by the horizontal axis and the Ga atom concentration in thesemiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

FIG. 29 shows the temperature dependence of a hole concentration foreach Ga atom concentration in a p-type diamond semiconductor accordingto embodiment 8 of the present invention. In FIG. 29 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Ga atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Ga atom concentration of 3.0×10¹⁶ cm⁻³ ormore is required. It is known that the deterioration in the quality ofthe diamond crystal occurs in the Ga atom concentration exceeding1.0×10²¹ cm⁻³. Accordingly, the dopant atom concentration in theGa-doped p-type diamond semiconductor element at a practical level isrequired to be no less than 3.0×10¹⁶ cm⁻³ and no more than 1.0×10²¹cm⁻³.

It is seen from this that, using equation (1), the ratio (Ga/C) of thenumber of Ga atoms to the number of C atoms in the source gas isrequired to fall within a range from no less than 0.3 ppm to no morethan 10⁴ ppm.

Using FIG. 29 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for each dopantatom concentration, a dopant atom concentration enabling the holeconcentration in the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³at each temperature can be found. For example, 8.1×10¹⁵ cm⁻³ is derivedfrom FIG. 29 as a Ga atom concentration enabling the hole concentrationin the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³ at 500K. Itis seen from equation (1) that the ratio (Ga/C) of the number of Gaatoms to the number of C atoms in the source gas in this case isrequired to fall within the range of from no less than 0.081 ppm to nomore than 10⁴ ppm.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the Ga atomconcentration in the p-type diamond semiconductor film, the higher thehole concentration, resulting in, needless to say, a diamondsemiconductor further suited to practical use.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is no less than 1.0×10¹⁵ cm⁻³ and the dopant atomconcentration is no more than 1.0×10²¹ cm⁻³. For this reason, the p-typediamond semiconductor according to embodiment 8 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Ga atom concentration 1.0×10²¹ cm⁻³. For example, embodiment8 can function as a p-type semiconductor even in any temperature otherthan 300K. The region in which the function as a p-type semiconductor isachieved in embodiment 8 is considerably wider than that in the case ofa conventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, Table 6 shows the mobility, at 300K, of holes in each of thep-type diamond semiconductor films obtained when trimethylgallium((CH₃)₃Ga: TMGa), triethylgallium ((C₂H₅)₃Ga: TEGa) or gallium chloride(GaCl₃) of the present invention is used as a dopant gas.

TABLE 6 (All in Ga atom concentration of 1.0 × 10¹⁹ cm⁻³) Ga dopantmaterial Hole mobility at room temperature (cm²/Vs) TMGa 1600 TEGa 1450GaCl₃ 200

As shown in Table 6, the room-temperature hole mobility in the case ofusing trimethylgallium ((CH₃)₃Ga: TMGa) or triethylgallium ((C₂H₅)₃Ga:TEGa) is about 7 times or more higher than that in the case of usinggallium chloride (GaCl₃), resulting in the diamond semiconductor havingnotably outstanding characteristics.

Embodiment 9

In a microwave plasma chemical vapor deposition technique, by using as afeedstock a gas mixture of a total flow rate of 300 ccm comprising areaction gas including a methane gas (CH₄) at a flow ratio of 1%, adopant gas and H² as the remainder, a diamond semiconductor film of thepresent invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. The pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW. Here, as the dopant gas, eithertrimethylindium ((CH₃)₃In: TMIn) or triethylindium ((C₂H₅)₃In: TEIn),which is an organometallic material including In, is used.

Hole measurement is conducted on the obtained diamond semiconductorfilms to evaluate the hole coefficient, whereby it can be confirmed thatthese diamond semiconductor films are p-type semiconductors.

Also, the In atom concentration in the diamond semiconductor film ismeasured by the SIMS measurement, and when the ratio of the number of Inatoms to the number of C atoms in the source gas (In/C)x(ppm) isrepresented by the horizontal axis and the In atom concentration in thesemiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

FIG. 30 shows the temperature dependence of a hole concentration foreach In atom concentration in a p-type diamond semiconductor accordingto embodiment 9 of the present invention. In FIG. 30 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each In atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, an In atom concentration of 1.5×10¹⁶ cm⁻³ ormore is required. It is known that the deterioration in the quality ofthe diamond crystal occurs in the In atom concentration exceeding1.0×10²¹ cm⁻³. Accordingly, the dopant atom concentration in theIn-doped p-type diamond semiconductor element at a practical level isrequired to be no less than 1.5×10¹⁶ cm⁻³ and no more than 1.0×10²¹cm⁻³.

It is seen from this that, using equation (1), a ratio (In/C) of thenumber of In atoms to the number of C atoms in the source gas isrequired to fall within a range of from no less than 0.15 ppm to no morethan 10⁴ ppm.

Using FIG. 30 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for each dopantatom concentration, a dopant atom concentration enabling the holeconcentration in the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³at each temperature can be found. For example, 5.1×10¹⁵ cm⁻³ is derivedfrom FIG. 30 as an In atom concentration enabling the hole concentrationin the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³ at 500K. Itis seen from equation (1) that the ratio (In/C) of the number of Inatoms to the number of C atoms in the source gas in this case isrequired to fall within the range of from no less than 0.051 ppm to nomore than 10⁴ ppm.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the In atomconcentration in the p-type diamond semiconductor film, the higher thehole concentration, resulting in, needless to say, a diamondsemiconductor further suited to practical use.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is 1.0×10¹⁵ cm⁻³ or more and the dopant atomconcentration is 1.0×10²¹ cm⁻³ or less. For this reason, the p-typediamond semiconductor according to embodiment 9 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the In atom concentration 1.0×10²¹ cm⁻³. For example, embodiment9 can function as a p-type semiconductor even in any temperature otherthan 300K. The region enabling functioning as a p-type semiconductor inembodiment 9 is considerably wider than that in the case of aconventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, Table 7 shows the mobility, at 300K, of holes in each of thep-type diamond semiconductor films obtained when trimethylindium((CH₃)₃In: TMIn), triethylindium ((C₂H₅)₃In: TEIn) or indium chloride(InCl₃) of the present invention is used as a dopant gas.

TABLE 7 (All in In atom concentration of 1.0 × 10¹⁹ cm⁻³) In dopantmaterial Hole mobility at room temperature (cm²/Vs) TMIn 1500 TEIn 1400InCl₃ 200

As shown in Table 7, the room-temperature hole mobility in the case ofusing trimethylindium ((CH₃)₃In: TMIn) or triethylindium ((C₂H₅)₃In:TEIn) is about 7 times or more higher than that in the case of usingindium chloride (InCl₃), resulting in the diamond semiconductor havingnotably outstanding characteristics.

Embodiment 10

Li may be used as dopant and an ion implantation technique may be usedto implant the Li dopant into a diamond single-crystal under theconditions of an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻²to produce Li-impurity-doped diamond. As an alternative to the ionimplantation technique, the Li-impurity-doped diamond may be produced bya microwave plasma chemical vapor deposition technique in whichsolid-state Li is introduced into plasma. By using as a feedstock areaction gas of a total flow rate of 300 ccm comprising a methane gas(CH₄) at a flow ratio of 1%, a dopant gas and H² as the remainder, adiamond semiconductor film of the present invention is grown to 1.0 μmthickness on a diamond single-crystal of (001) surface orientation.Here, the pressure in the reaction tube is 50 Torr and the microwavesource has a frequency of 2.45 GHz and a power of 1.3 kW. Here, as thedopant gas, any of the three, methyllithium (CH₃Li), ethyllithium(C₂H₅Li) and propyllithium (C₃H₇Li), may be used.

Then, annealing is performed on the Li-impurity-doped diamond thusobtained. Hole measurement is conducted on the Li-doped diamondsemiconductor films of the present invention thus obtained to evaluatethe hole coefficient, whereby it can be confirmed that these diamondsemiconductor film is a p-type semiconductor. In addition, the Li dopantatom concentration in the diamond semiconductor film is measured by theSIMS measurement.

Through the hole measurement, the temperature dependence of the holeconcentration in a sample of each of the Li dopant atom concentrationsfrom 1.0×10¹⁶ cm⁻³ to 1.0×10²¹ cm⁻³, is measured. From this result, FIG.39 shows the temperature dependence of a hole concentration for each Liatom concentration in the p-type diamond semiconductor according toembodiment 10 of the present invention. In FIG. 39 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Li atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Li atom concentration of 3.0×10¹⁷ cm⁻³ ormore is required. It is known that deterioration in the quality of thediamond crystal occurs in a Li atom concentration exceeding 1.0×10²¹cm⁻³. Accordingly, the dopant atom concentration in the Li-doped p-typediamond semiconductor element at about 300K in the practical level isrequired to be no less than 3.0×10¹⁷ cm⁻³ and no more than 1.0×10²¹cm⁻³.

Using FIG. 39 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for eachconcentration of the dopant atoms, a dopant atom concentration enablingthe hole concentration in the p-type diamond semiconductor to reach1.0×10¹⁵ cm⁻³ at each temperature can be found. For example, 3.2×10¹⁶cm⁻³ is derived from FIG. 39 as a Li atom concentration enabling thehole concentration in the p-type diamond semiconductor to reach 1.0×10¹⁵cm⁻³ at 500K.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the Li atomconcentration, the higher the hole concentration, resulting in, needlessto say, a diamond semiconductor further suited to practical use.

Also, in the P-type diamond semiconductor according to the embodiment itis possible to obtain a room-temperature hole concentration about 5700times that in a conventional p-type diamond semiconductor doped with B,with the same dopant atom concentration.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is 1.0×10¹⁵ cm⁻³ or more and the dopant atomconcentration is 1.0×10²¹ cm⁻³ or less. For this reason, the p-typediamond semiconductor according to embodiment 10 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Li atom concentration 1.0×10²¹ cm⁻³. For example, embodiment10 can function as a p-type semiconductor even in any temperature otherthan 300K. The region enabling functioning as a p-type semiconductor inembodiment 10 is considerably wider than that in the case of aconventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

TABLE 8 (All in Li atom concentration of 1.0 × 10¹⁹ cm⁻³) Li dopantmaterial Hole mobility at room temperature (cm²/Vs) Methyl Li 1350 EthylLi 1250 Propyl Li 150

Embodiment 11

In a microwave plasma chemical vapor deposition technique, by using as afeedstock a gas mixture of a total flow rate of 300 ccm comprising areaction gas including a methane gas (CH₄) at a flow ratio of 1%, adopant gas and H² as the remainder, a diamond semiconductor film of thepresent invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. The pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW. Here, as the dopant gas,bis-cyclopentadienyl magnesium ((C₅H₅)₂Mg: Cp₂Mg) orbis-methylcyclopentadienyl magnesium ((CH₃C₅H₄)₂Mg: MCp₂Mg), which is anorganometallic material including Mg, is used.

Hole measurement is conducted on the obtained diamond semiconductorfilms to evaluate the hole coefficient, whereby it can be confirmed thatthese diamond semiconductor films are p-type semiconductors.

Also, the Mg atom concentration in the diamond semiconductor film ismeasured by the SIMS measurement, and when the ratio of the number of Mgatoms to the number of C atoms in the source gas (Mg/C)x(ppm) isrepresented by the horizontal axis and the Mg atom concentration in thesemiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

FIG. 31 shows the temperature dependence of a hole concentration foreach Mg atom concentration in a p-type diamond semiconductor accordingto embodiment 11 of the present invention. In FIG. 31 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Mg atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Mg atom concentration of 2.0×10¹⁸ cm⁻³ ormore is required. It is known that deterioration in the quality of thediamond crystal occurs in the Mg atom concentration exceeding 1.0×10²¹cm⁻³. Accordingly, the dopant atom concentration in the Mg-doped p-typediamond semiconductor element at a practical level is required to be noless than 2.0×10¹⁸ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

It is seen from this that, using equation (1), the ratio (Mg/C) of thenumber of Mg atoms to the number of C atoms in the source gas isrequired to fall within a range of from no less than 20 ppm to no morethan 10⁴ ppm.

Using FIG. 31 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for each dopantatom concentration, the dopant atom concentration enabling the holeconcentration in the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³at each temperature can be found. For example, 1.0×10¹⁷ cm⁻³ is derivedfrom FIG. 31 as an Mg atom concentration enabling the hole concentrationin the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³ at 500K. Itis seen from equation (1) that the ratio (Mg/C) of the number of Mgatoms to the number of C atoms in the source gas in this case isrequired to fall within the range of from no less than 1.0 ppm to nomore than 10⁴ ppm.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the Mg atomconcentration in the p-type diamond semiconductor film, the higher thehole concentration is, resulting in, needless to say, a diamondsemiconductor further suited to practical use.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is 1.0×10¹⁵ cm⁻³ or more and the dopant atomconcentration is 1.0×10²¹ cm⁻³ or less. For this reason, the p-typediamond semiconductor according to embodiment 11 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Mg atom concentration 1.0×10²¹ cm⁻³. For example, embodiment11 can function as a p-type semiconductor even in any temperature otherthan 300K. The region enabling functioning as a p-type semiconductor inembodiment 11 is considerably wider than that in the case of aconventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, Table 9 shows the mobility, at 300K, of holes in each of thep-type diamond semiconductor films obtained when bis-cyclopentadienylmagnesium ((C₅H₅)₂Mg: Cp₂Mg), bis-methylcyclopentadienyl magnesium((CH₃C₅H₄)₂Mg: MCp₂Mg) or magnesium chloride (MgCl₂) of the presentinvention is used as a dopant gas.

TABLE 9 (All in Mg atom concentration of 1.0 × 10¹⁹ cm⁻³) Mg dopantmaterial Hole mobility at room temperature (cm²/Vs) Cp2Mg 1350 MCp2Mg1250 MgCl₂ 100

As shown in Table 9, the room-temperature hole mobility in the case ofusing bis-cyclopentadienyl magnesium ((C₅H₅)₂Mg: Cp₂Mg) andbis-methylcyclopentadienyl magnesium ((CH₃C₅H₄)₂Mg: MCp₂Mg) is about 12times or more higher than that in the case of using magnesium chloride(MgCl₂), resulting in the diamond semiconductor having notablyoutstanding characteristics.

Embodiment 12

In a microwave plasma chemical vapor deposition technique, by using as afeedstock a gas mixture of a total flow rate of 300 ccm comprising areaction gas including a methane gas (CH₄) at a flow ratio of 1%, adopant gas and H² as the remainder, a diamond semiconductor film of thepresent invention is grown to 1.0 μm thickness on a diamondsingle-crystal of (001) surface orientation. The pressure in thereaction tube is 50 Torr and the microwave source has a frequency of2.45 GHz and a power of 1.3 kW. Here, as the dopant gas, dimethylzinc((CH₃)₂Zn: DMZn) or diethylzinc ((C₂H₅)₂Zn: DEZn), which is anorganometallic material including Zn, is used.

Hole measurement is conducted on the diamond semiconductor films thusobtained to evaluate the hole coefficient, whereby it can be confirmedthat these diamond semiconductor films are p-type semiconductors.

Also, the Zn atom concentration in the diamond semiconductor film ismeasured by the SIMS measurement, and when the ratio of the number of Znatoms to the number of C atoms in the source gas (Zn/C)x(ppm) isrepresented by the horizontal axis and the Zn atom concentration in thesemiconductor film y(cm⁻³) is represented by the vertical axis, therelationship is, as shown in FIG. 24,X(ppm)×10¹⁷ =y(cm⁻³)  (1)

FIG. 32 shows the temperature dependence of a hole concentration foreach Zn atom concentration in a p-type diamond semiconductor accordingto embodiment 12 of the present invention. In FIG. 32 measurementtemperature (K) is represented by the horizontal axis and the holeconcentration (cm⁻³) in the p-type diamond semiconductor is representedby the vertical axis, and the measured values for each Zn atomconcentration (cm⁻³) in the p-type diamond semiconductor are plotted.

In order to obtain a hole concentration of 1.0×10¹⁵ cm⁻³ at about 300Kwhich is a practical level, a Zn atom concentration of 1.0×10¹⁷ cm⁻³ ormore is required. It is known that deterioration in the quality of thediamond crystal occurs in the Zn atom concentration exceeding 1.0×10²¹cm⁻³. Accordingly, the dopant atom concentration in the Zn-doped p-typediamond semiconductor element at a practical level is required to be noless than 1.0×10¹⁷ cm⁻³ and no more than 1.0×10²¹ cm⁻³.

It is seen from this that, using equation (1), the ratio (Zn/C) of thenumber of Zn atoms to the number of C atoms in the source gas isrequired to fall within a range of from no less than 1.0 ppm to no morethan 10⁴ ppm.

Using FIG. 32 which is the plot of the values of the hole concentrationin the p-type diamond semiconductor at each temperature for each dopantatom concentration, a dopant atom concentration enabling the holeconcentration in the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³at each temperature can be found. For example, 1.6×10¹⁶ cm⁻³ is derivedfrom FIG. 32 as a Zn atom concentration enabling the hole concentrationin the p-type diamond semiconductor to reach 1.0×10¹⁵ cm⁻³ at 500K. Itis seen from equation (1) that the ratio (Zn/C) of the number of Znatoms to the number of C atoms in the source gas in this case isrequired to fall within the range of from no less than 0.16 ppm to nomore than 10⁴ ppm.

In the range of 1.0×10²¹ cm⁻³ or less, the higher the Zn atomconcentration in the p-type diamond semiconductor film, the higher thehole concentration, resulting in, needless to say, a diamondsemiconductor further suited to practical use.

Also, the requirement for functioning as a p-type semiconductor is thatthe hole concentration is 1.0×10¹⁵ cm⁻³ or more and the dopant atomconcentration is 1.0×10²¹ cm⁻³ or less. For this reason, the p-typediamond semiconductor according to embodiment 12 functions as a p-typesemiconductor under the conditions of being included in the regionenclosed by the line of the hole concentration 1.0×10¹⁵ cm⁻³ and theline of the Zn atom concentration 1.0×10¹⁵ cm⁻³. For example, embodiment12 can function as a p-type semiconductor even in any temperature otherthan 300K. The region in which the function as a p-type semiconductor isachieved in embodiment 12 is considerably wider than that in the case ofa conventional p-type diamond semiconductor doped with B, resulting insuperiority in function as a p-type semiconductor under variousconditions.

Also, Table 10 shows the mobility, at 300K, of holes in each of thep-type diamond semiconductor films obtained when dimethylzinc ((CH₃)₂Zn:DMZn), diethylzinc ((C₂H₅)₂Zn: DEZn) or zinc chloride (ZnCl₂) used inthe present invention is used as a dopant gas.

TABLE 10 (All in Zn atom concentration of 1.0 × 10¹⁹ cm⁻³) Zn dopantmaterial Hole mobility at room temperature (cm²/Vs) DMZn 1200 DEZn 1150ZnCl₂ 50

As shown in Table 10, the room-temperature hole mobility in the case ofusing dimethylzinc ((CH₃)₂Zn: DMZn) or diethylzinc ((C₂H₅)₂Zn: DEZn) isabout 23 times or more higher than that in the case of using zincchloride (ZnCl₂), resulting in the diamond semiconductor having notablyoutstanding characteristics.

Embodiment 13

Diamond powder is mixed with Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn asdopant and the mixture is dissolved in a Fe—Ni solution. It is placedfor 7 hours under the conditions of 5.0 GPa and about 1.4×10³° C., withthe result that a p-type diamond semiconductor film of the presentinvention can be obtained by the ultra-high-temperature andhigh-pressure technique. Hole measurement is conducted on the p-typediamond semiconductor films of the present invention thus obtained toevaluate the hole coefficient, whereby it can confirmed that thesep-type diamond semiconductor films are p-type semiconductors.

Also, Table 11 shows the hole concentration and the hole mobility in thep-type diamond semiconductor film at 300K when the dopant atomconcentrations in the diamond semiconductor films are measured by theSIMS measurement and the ratio of the atom concentration (dopant atom/C)is equally fixed at 0.01%.

TABLE 11 Room-temperature hole Dopant concentration (cm⁻³) Hole mobility(cm²/(V · s)) B 6.2 × 10¹² 200 (conventional art) Al 4.5 × 10¹⁶ 960 Be1.4 × 10¹⁷ 1000 Ca 3.0 × 10¹⁶ 1150 Cd 4.5 × 10¹⁷ 970 Ga 3.1 × 10¹⁷ 1000In 6.7 × 10¹⁷ 950 Li 3.0 × 10¹⁶ 1150 Mg 4.4 × 10¹⁵ 1100 Zn 9.7 × 10¹⁶1130

The room-temperature hole concentration in the case of doping Al, Be,Ca, Cd, Ga, In, Li, Mg or Zn is 4.8×10³ to 1.1×10⁵ times higher thanthat in the case of doping B in the conventional art (6.2×10¹² cm⁻³),and notably outstanding. Also, the room-temperature hole mobility in thecase of doping Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn is 4.7 to 5.7 timeshigher than that in the case of doping B in a conventional art (200cm²/(Vs)), and notably outstanding.

Embodiment 14

Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn is used as dopant and an ionimplantation technique is used to implant the dopant into a diamondsingle-crystal under the conditions of an acceleration voltage of 150 kVand a dose of 10¹⁵ cm⁻² to produce impurity-doped diamond. Then, thediamond thus obtained is annealed. Hole measurement is conducted on theimpurity-doped diamond semiconductor films of the present invention thusobtained to evaluate the hole coefficient, whereby it can be confirmedthat these diamond semiconductor films are p-type semiconductors.

Also, Table 12 shows the hole concentration and the hole mobility in thep-type diamond semiconductor film at 300K when the dopant atomconcentrations in the diamond semiconductor films are measured by theSIMS measurement and the dopant atom concentration is equally fixed at1.0×10¹⁹ cm⁻³.

TABLE 12 Room-temperature hole Dopant concentration (cm⁻³) Hole mobility(cm²/(V · s)) B 3 × 10¹² 50 (conventional art) Al 2 × 10¹⁶ 900 Be 7 ×10¹⁶ 950 Ca 2 × 10¹⁶ 1050 Cd 4 × 10¹⁷ 940 Ga 2 × 10¹⁷ 950 In 4 × 10¹⁷930 Li 2 × 10¹⁶ 1050 Mg 3 × 10¹⁵ 1050 Zn 8 × 10¹⁶ 1030

The room-temperature hole concentration in the case of doping Al, Be,Ca, Cd, Ga, In, Li, Mg or Zn is 1.0×10³ to 6.7×10⁵ times higher thanthat in the case of doping B in the conventional art (3.0×10¹² cm⁻³),and notably outstanding. On the other hand, the room-temperature holemobility in the case of doping Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn is18 to 21 times higher than that in the case of doping B in theconventional art (50 cm²/(Vs)), and notably outstanding.

Embodiment 15

FIG. 33 illustrates a structural cross-section view of an MESFET(Metal-Semiconductor Field-Effect Transistor) according to embodiment 15of the present invention. In a microwave plasma chemical vapordeposition technique, by using as a feedstock a gas mixture of a totalflow rate of 300 ccm comprising a reaction gas including a methane gas(CH₄) at a flow ratio of 1%, a dopant gas and H² as the remainder, ap-type diamond semiconductor film 4-12 is grown to 1.0 μm thickness on adiamond substrate 4-11. In the embodiment, the pressure in the reactiontube is 50 Torr and the microwave source has a frequency of 2.45 GHz anda power of 1.3 kW. Here, as the dopant, Al, Ca, Cd, Ga, In, Li, Mg or Znis used.

In the use of Be as dopant, by an ion implantation technique, the Bedopant is implanted into a diamond single-crystal under the conditionsof an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻² to producethe p-type diamond semiconductor film 4-12.

To achieve electrical insulation between transistors, in the areaoutside the transistor, the p-type diamond semiconductor film 4-12 isetched away until the diamond substrate 4-11 is exposed. Specifically,the etching is carried out such that the p-type diamond semiconductorfilm 4-12 has the area of the surface in the direction at right anglesto the stacking direction reduced so as to be smaller than the diamondsubstrate 4-11, with the portion located around the center of thediamond substrate 4-11 still remaining.

On the p-type diamond semiconductor film 4-12, gold (Au) is evaporatedas a source electrode 4-13, Al as a gate electrode 4-14 and Au as adrain electrode 4-15 to produce a FET.

Table 13 shows transconductance (gm) (amplification factor) at 300K, ofa conventional MESFET and the MESFET according to embodiment 15 of thepresent invention, in which the dopant atom concentration of the p-typediamond semiconductor film 4-12 is 1.0×10¹⁸ cm⁻³.

TABLE 13 Dopant Transconductance (mS/mm) B (conventional art) 1.0 × 10⁻⁴Al 1.2 × 10² Be 1.5 × 10² Ca 1.1 × 10² Cd 9.0 × 10 Ga 8.0 × 10 In 1.0 ×10² Li 1.1 × 10² Mg 1.2 × 10² Zn 9.0 × 10

In the conventional MESFET having the B-doped diamond semiconductorfilm, the gm=0.0001 mS/mm, whereas the MESFET having the p-type diamondsemiconductor film doped with Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn has agm 8.0×10⁵ to 1.5×10⁶ times higher, which is notably outstanding.

The gm values of the MESFET according to embodiment 15 shown in Table 13are the results obtained by being provided with the p-type diamondsemiconductor film 4-12 produced by the microwave plasma chemical vapordeposition technique. In the case of using the ion implantation or thehigh-temperature and high-pressure synthesis technique for the processof producing the p-type diamond semiconductor film 4-12, thetransconductance (gm) decreases so as to be half of that in embodiment15, but is still highly outstanding as compared with that in theconventional MEFET.

Embodiment 16

FIG. 34 illustrates a structural cross-section view of an MISFET(Metal-Insulating film-Semiconductor Field-Effect Transistor) accordingto embodiment 16 of the present invention. In a microwave plasmachemical vapor deposition technique, by using as a feedstock a gasmixture of a total flow rate of 300 ccm comprising a reaction gasincluding a methane gas (CH₄) at a flow ratio of 1%, a dopant gas and H²as the remainder, a diamond semiconductor film 4-22 is grown to 1.0 μmthickness on a diamond substrate 4-21. In the embodiment, the pressurein the reaction tube is 50 Torr and the microwave source has a frequencyof 2.45 GHz and a power of 1.3 kW. Here, as the dopant in the p-typediamond semiconductor film 4-22, Al, Ca, Cd, Ga, In, Li, Mg or Zn isused.

In the use of Be as the dopant, by an ion implantation technique, the Bedopant is implanted into a diamond single-crystal under the conditionsof an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻² to producethe p-type diamond semiconductor film 4-22.

To achieve electrical insulation between transistors, the peripheralportion of the diamond semiconductor film 4-22 is etched away until thediamond substrate 4-21 is exposed. Specifically, the etching is carriedout such that the diamond semiconductor film 4-22 has the area of thesurface in the direction at right angles to the stacking directionreduced so as to be smaller than the diamond substrate 4-21, with theportion located around the center of the diamond substrate 4-21 stillremaining.

On the diamond semiconductor film 4-22, Au is evaporated as a sourceelectrode 4-23, Au as a drain electrode 4-24, SiO₂ as an insulating film4-25 in a gate area and Al as a gate electrode 4-26 to produce a FET.

Table 14 shows transconductance (gm) (amplification factor) at 300K, ofa conventional MESFET and the MISFET according to embodiment 16 of thepresent invention, in which the dopant atom concentration of the diamondsemiconductor film 4-22 is 1.0×10¹⁸ cm⁻³.

TABLE 14 Dopant Transconductance (mS/mm) B (conventional art) 1.0 × 10⁻⁵Al 1.1 × 10² Be 1.3 × 10² Ca 1.0 × 10² Cd 8.5 × 10 Ga 7.5 × 10 In 9.0 ×10 Li 1.0 × 10² Mg 8.0 × 10 Zn 6.5 × 10

In the conventional MISFET having the B-doped p-type diamondsemiconductor film, the gm=1.0×10⁻⁵ mS/mm, whereas the MISFET having thediamond semiconductor film doped with Al, Be, Ca, Cd, Ga, In, Li, Mg orZn has a gm 7.5×10⁶ to 1.3×10⁷ times, which is notably outstanding.

The gm values of the MISFET according to embodiment 12 shown in Table 14are the results obtained by being provided with the diamondsemiconductor film 4-22 produced by the microwave plasma chemical vapordeposition technique. In the case of using the ion implantation or thehigh-temperature and high-pressure synthesis technique for the processof producing the diamond semiconductor film 4-22, the transconductance(gm) decreases so as to be half of that in embodiment 12, but is stillhighly outstanding as compared with that in the conventional MEFET.

Embodiment 17

FIG. 35 illustrates a structural cross-section view of an npn-typebipolar transistor according to embodiment 17 of the present invention.In a microwave plasma chemical vapor deposition technique, a gas mixtureof a total flow rate of 300 ccm comprising a reaction gas including amethane gas (CH₄) at a flow ratio of 1%, a dopant gas and H² as theremainder is used as a feedstock, to grow, on a diamond substrate 4-31,in order, an n-type diamond semiconductor film 4-32 of 5.0 μm thicknessand then a p-type diamond semiconductor film 4-33 and an n-type diamondsemiconductor film 4-34 which are of 0.5 μm thickness. In theembodiment, the pressure in the reaction tube is 50 Torr and themicrowave source has a frequency of 2.45 GHz and a power of 1.3 kW.Here, as the dopant in the p-type semiconductor film 4-33, Al, Ca, Cd,Ga, In, Li, Mg or Zn is used.

In the use of Be as the dopant, by an ion implantation technique, the Bedopant is implanted into a diamond single-crystal under the conditionsof an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻² to producethe p-type diamond semiconductor film 4-33.

To achieve electrical insulation between transistors, the peripheralportion of the n-type diamond semiconductor film 4-32 is etched awayuntil the diamond substrate 4-31 is exposed. Also, for formingelectrodes, the p-type diamond semiconductor film 4-33 and the n-typediamond semiconductor film 4-34 are etched away as shown in FIG. 35. Tiis evaporated as a collector electrode 4-35 on the n-type diamondsemiconductor film 4-32, Ni as a base electrode 4-36 on the p-typediamond semiconductor film 4-33, and Ti as an emitter electrode 4-37 onthe n-type diamond semiconductor film 4-34.

Table 15 shows a current amplification factor β at 300K, of aconventional npn-type bipolar transistor and the npn-type bipolartransistor according to embodiment 17 of the present invention, in whichthe dopant atom concentration of the p-type diamond semiconductor film4-33 is 1.0×10¹⁸ cm⁻³.

TABLE 15 Dopant Current amplification factor β B (conventional art) 1.0× 10⁻² Al 2.0 × 10³ Be 5.0 × 10² Ca 2.0 × 10³ Cd 1.6 × 10³ Ga 1.4 × 10³In 1.2 × 10³ Li 2.0 × 10³ Mg 1.6 × 10³ Zn 1.4 × 10³

In the conventional npn-type bipolar transistor having the B-dopedp-type diamond semiconductor film, the β=1.0×10⁻², whereas the npn-typebipolar transistor having the p-type diamond semiconductor film dopedwith Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn has β 5.0×10⁴ to 2.0×10⁵times, which is notably outstanding.

The current amplification factors β of the npn-type bipolar transistoraccording to embodiment 17 shown in Table 15 are the results obtained bybeing provided with the p-type diamond semiconductor film 4-33 producedusing the microwave plasma chemical vapor deposition technique. In thecase of using the ion implantation or the high-temperature andhigh-pressure synthesis technique for the process of producing thep-type diamond semiconductor film 4-33, the current amplification factorβ decreases so as to be half of that in embodiment 17, but is stillhighly outstanding as compared with that in the conventional npn-typebipolar transistor.

Embodiment 18

FIG. 36 illustrates a structural cross-section view of a pnp-typebipolar transistor according to embodiment 18 of the present invention.In a microwave plasma chemical vapor deposition technique, a gas mixtureof a total flow rate of 300 ccm comprising a reaction gas including amethane gas (CH₄) at a flow ratio of 1%, a dopant gas and H² as theremainder is used as a feedstock, to grow, on a diamond substrate 4-41,in order, a p-type diamond semiconductor film 4-42 of 5.0 μm thickness,and an n-type diamond semiconductor film 4-43 and a p-type diamondsemiconductor film 4-44 which are of 0.5 μm thickness. In theembodiment, the pressure in the reaction tube is 50 Torr and themicrowave source has a frequency of 2.45 GHz and a power of 1.3 kW.Here, as the dopant in the p-type semiconductor films 4-42 and 4-44, Al,Ca, Cd, Ga, In, Li, Mg or Zn is used.

In the use of Be as the dopant, by an ion implantation technique, the Bedopant is implanted into a diamond single-crystal under the conditionsof an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻² to producethe p-type diamond semiconductor films 4-42 and 4-44.

To achieve electrical insulation between transistors, the peripheralportion of the p-type diamond semiconductor film 4-42 is etched awayuntil the diamond substrate 4-41 is exposed. Also, for formingelectrodes, the n-type diamond semiconductor film 4-43 and the p-typediamond semiconductor film 4-44 are etched away as shown in FIG. 36. Niis evaporated as a collector electrode 4-45 on the p-type diamondsemiconductor film 4-42, Ti as a base electrode 4-46 on the n-typediamond semiconductor film 4-43, and Ni as an emitter electrode 4-47 onthe p-type diamond semiconductor film 4-44.

Table 16 shows a current amplification factor β at 300K, of aconventional pnp-type bipolar transistor and the pnp-type bipolartransistor according to embodiment 18 of the present invention, in whichthe dopant atom concentration of the p-type diamond semiconductor films4-42 and 4-44 is 1.0×10¹⁸ cm⁻³.

TABLE 16 Dopant Current amplification factor β B (conventional art) 1.0× 10⁻³ Al 1.7 × 10³ Be 4.0 × 10² Ca 1.8 × 10³ Cd 1.5 × 10³ Ga 1.3 × 10³In 1.2 × 10³ Li 1.8 × 10³ Mg 1.5 × 10³ Zn 1.3 × 10³

In the conventional pnp-type bipolar transistor having the B-dopedp-type diamond semiconductor film, the β=1.0×10⁻³, whereas the pnp-typebipolar transistor having the p-type diamond semiconductor film dopedwith Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn has β4.0×10⁵ to 1.8×10⁶ times,which is notably outstanding.

The current amplification factors β of the pnp-type bipolar transistoraccording to embodiment 18 shown in Table 16 are the results obtained bybeing provided with the p-type diamond semiconductor films 4-42 and 4-44produced using the microwave plasma chemical vapor deposition technique.In the case of using the ion implantation or the high-temperature andhigh-pressure synthesis technique for the process of producing thep-type diamond semiconductor films 4-42 and 4-44, the currentamplification factor β decreases so as to be half of that in embodiment18, but is still highly outstanding as compared with that in theconventional pnp-type bipolar transistor.

Embodiment 19

FIG. 37 illustrates a structural cross-section view of a light emittingdiode (LED) according to embodiment 19 of the present invention. In amicrowave plasma chemical vapor deposition technique, a gas mixture of atotal flow rate of 300 ccm comprising a reaction gas including a methanegas (CH₄) at a flow ratio of 1%, a dopant gas and H² as the remainder isused as a feedstock, to grow, on a diamond substrate 4-51, in order, ap-type diamond semiconductor film 4-52 of 5.0 μm thickness and an n-typediamond semiconductor film 4-53 of 0.5 μm thickness. In the embodiment,the pressure in the reaction tube is 50 Torr and the microwave sourcehas a frequency of 2.45 GHz and a power of 1.3 kW. Here, as the dopantin the p-type semiconductor film 4-52, Al, Ca, Cd, Ga, In, Li, Mg or Znis used.

In the use of Be as the dopant, by an ion implantation technique, the Bedopant is implanted into a diamond single-crystal under the conditionsof an acceleration voltage of 150 kV and a dose of 10¹⁵ cm⁻² to producethe p-type diamond semiconductor film 4-52.

To achieve electrical insulation between LEDs, the p-type diamondsemiconductor film 4-52 and the n-type diamond semiconductor film 4-53are etched away as illustrated in FIG. 37. Also, an Ni anode electrode4-54 is evaporated on the p-type diamond semiconductor film 4-52, and aTi cathode electrode 4-55 is evaporated on the n-type diamondsemiconductor film 4-53.

Table 17 shows emission intensities (output power density) at awavelength of 235 nm of a conventional light emitting diode (LED) andthe LED according to embodiment 19 of the present invention when theapplied voltage is 7V and the current density is 10 A/mm².

TABLE 17 Dopant Output power density (mW/mm²) B (conventional art) 1.0 ×10⁻⁴ Al 5.0 × 10³ Be 4.0 × 10³ Ca 2.8 × 10³ Cd 4.0 × 10³ Ga 7.1 × 10³ In5.0 × 10³ Li 2.8 × 10³ Mg 4.7 × 10³ Zn 4.7 × 10³

It is seen that the output power density in the case of using B in theconventional art is 1.0×10⁻⁴ mW/mm², but the output power density in thecase of using Al, Be, Ca, Cd, Ga, In, Li, Mg or Zn is 2.8×10⁷ to 7.1×10⁷times higher, which is notably outstanding.

In FIG. 37, the p-type diamond semiconductor film 4-52 and the n-typediamond semiconductor film 4-53 are stacked in this order, but even inthe reversed order the characteristics are not changed. In other words,the same results as those in Table 17 are obtained equally in an LEDhaving an n-type diamond semiconductor film and a p-type diamondsemiconductor film stacked in this order on the diamond substrate 4-51.

The emission intensities (output power density) at a wavelength of 235nm of LED according to embodiment 19 shown in Table 17 are the resultsobtained by being provided with the p-type diamond semiconductor film4-52 produced using the microwave plasma chemical vapor depositiontechnique. In the case of using the ion implantation or thehigh-temperature and high-pressure synthesis technique for the processof producing the p-type diamond semiconductor film 4-52, the emissionintensity (output power density) at a wavelength of 235 nm decreases soas to be approximately half, but the LED still has outstandingcharacteristics as compared with that in the conventional LED.

The present invention attaches importance to the use of any of thefollowing nine, Al, Be, Ca, Cd, Ga, In, Li, Mg and Zn as dopant in ap-type diamond semiconductor, and is not limited to the process ofproducing the p-type diamond semiconductor film used in each of theembodiments.

Next, a process for producing a diamond semiconductor according to thepresent invention in order to attain the fifth object will be describedin detail.

The present invention makes it possible to produce a high qualitydiamond semiconductor by providing a protective layer to protect anion-implanted diamond thin-film formed on the surface beforehigh-temperature and high-pressure annealing is performed so as to makeit possible to eliminate damage caused by ion implantation whilepreventing the surface from being etched by the high-temperature andhigh-pressure annealing.

Embodiment 20

FIGS. 40A to 40G illustrate the process-steps in producing a diamondsemiconductor according to embodiment 20 of the present invention. Adiamond substrate 5-11 is prepared (FIG. 40A). A microwave plasma CVDapparatus is used to laminate a diamond thin-film 5-12 of 1 μm on thediamond substrate 5-11 at a substrate temperature of 700° C. usingmethane as a reaction gas (FIG. 40B). The embodiment employs themicrowave plasma CVD technique, but any technique can be employed aslong as the diamond thin-film 5-12 is formed. A diamond single crystalproduced by high-temperature and high-pressure synthesis may be used.

An ion implanting apparatus is used to implant dopant into the diamondthin-film 5-12 at an acceleration voltage of 60 kV and a dose of 1×10¹⁴cm⁻² (FIGS. 40C, 40D). Here, as the implanted dopant, we have B, Al, Ga,In, Zn, Cd, Be, Mg, Ca, P, As, Sb, O, S, Se, Li, Na, and K.

Then, a protective layer (platinum) 5-14 is formed on the ion-implanteddiamond thin-film 5-13 (FIG. 40E). In the embodiment, platinum is usedas the protective layer 5-14, but the protective layer 5-14 may be amultilayer film of various metals, alloy, oxides, nitrides andcombinations of these having a thin-film of 0.01 μm to 10 μm. Inparticular, the protective layer 5-14 is desirably designed as a layerof a metal including at least one of the five, titanium, tungsten,platinum, palladium and molybdenum, or a layer ofAl_(1-x)Si_(x)O_(1-y)N_(y ()0≦x≦1, 0≦y≦1), or a layer comprising manylayers of no less than two of them. The protective layer 5-14 may beproduced by vapor deposition, sputtering, CVD technique, laser ablationtechnique.

The ion implanted diamond thin-film 5-13 with the protective layer 5-14formed thereon is placed in an ultra-high-temperature and high-pressurefiring furnace, and is annealed at a pressure and a temperature of noless than 3.5 GPa and no less than 600° C. (FIG. 40F). In other words,the annealing is carried out under the conditions of the pressure P(kbar) and the temperature T (K) being a pressure of no less than 35kbar and a temperature of no less than 873K which satisfy the relationof the expression P>7.1+0.027 T (see non-patent document 7).

The protective layer 5-14 is removed by acid, and then a semiconductordiamond thin-film 5-15 is obtained (FIG. 40G).

By way of example, after electrodes have been formed on thesemiconductor diamond thin-film 5-15 which is produced through theannealing for one hour under the conditions of 1400° C. and 7 GPa, holemeasurement is conducted on the thin-film 5-15 to determine polarity,carrier concentration at room temperature and mobility at roomtemperature. Table 18 shows the polarity, the carrier concentration andmobility at room temperature for each dopant of the semiconductordiamond thin-film 5-15 in this example.

TABLE 18 Room-temperature Room-temperature carrier mobility DopantPolarity concentration (cm⁻³) (cm²/Vs) B P-type 3 × 10¹³ 1100 Al P-type5 × 10¹⁵ 1000 Ga P-type 7 × 10¹⁶ 900 In P-type 6 × 10¹⁶ 800 Zn P-type 5× 10¹⁶ 900 Cd P-type 6 × 10¹⁶ 950 Be P-type 7 × 10¹⁷ 1000 Mg P-type 8 ×10¹⁷ 1150 Ca P-type 2 × 10¹⁷ 800 P N-type 3 × 10¹¹ 650 As N-type 7 ×10¹⁵ 700 Sb N-type 5 × 10¹⁵ 950 O N-type 3 × 10¹³ 250 S N-type 6 × 10¹³800 Se N-type 5 × 10¹⁴ 700 Li N-type 6 × 10¹⁴ 600 Na N-type 3 × 10¹⁴ 650K N-type 5 × 10¹⁴ 500

As shown in Table 18, the semiconductor diamond thin-film 5-15 has thecharacteristics of a high-quality P-type/N-type semiconductor which areunable to be provided by conventional producing processes, as a resultof forming the protective layer 5-14 on the ion implanted diamondthin-film 5-13 before performing the high-temperature and high-pressureannealing.

Further, FIG. 42 shows the cathode luminescence (CL) spectra(measurement temperature: 10K) before and after high-temperature andhigh-pressure annealing is performed on a diamond thin-film implantedwith boron (B) ions as dopant by way of example. In the ion implanteddiamond thin-film 5-13 before the high-temperature and high-pressureannealing, the light-emission in relation to free exciton (FE) peculiarto a diamond is not observed. However, FE-related light-emission occursin the semiconductor diamond thin-film 5-15 after high-temperaturehigh-pressure annealing under the conditions of 1400° C. and 7 GPa. ThisFE light-emission results from exciton, and the emission intensityincreases with an increase in the crystal quality. As a result, thecrystal quality can be evaluated with reference to the emissionintensity. Also, from the observation of bound-exciton light-emission(BE) originating in boron, it is seen that boron functions as dopant inthe diamond thin-film.

These facts demonstrate that the crystal quality of the semiconductordiamond thin-film 5-15 so produced is improved as compared with that ofthe ion implanted diamond thin-film 5-13 before the high-temperature andhigh-pressure annealing, and that boron exists in the semiconductordiamond thin-film 5-15. Specifically, they demonstrate that, byimplanting boron ions, crystal defects and an amorphous layer areintroduced into the ion implanted diamond thin-film 5-13, resulting inthe occurrence of deterioration of the diamond crystal, but, by thehigh-temperature and high-pressure annealing, these crystal defects andamorphous layer are removed and boron is activated as dopant.

As described above, the results of CL also suggest that thesemiconductor diamond thin-film 5-15 obtained by the process ofproducing the diamond semiconductor according to the embodiment is ahigh-quality P-type/N-type diamond semiconductor such as is unable to beprovided by conventional processes.

Embodiment 21

FIGS. 41A to 41F illustrate the process-steps in producing a diamondsemiconductor according to embodiment 21 of the present invention. Adiamond substrate 5-21 is prepared (FIG. 41A). A microwave plasma CVDapparatus is used to laminate a diamond thin-film of 1 μm onto thediamond substrate 5-21 at a substrate temperature of 700° C. usingmethane as a reaction gas (FIG. 41B).

After that, an ion implanting apparatus is used to implant dopant intothe diamond thin-film 5-22 thus produced at an acceleration voltage of60 kV and a dose of 1×10¹⁴ cm⁻² (FIGS. 41C, 41D). As in the case ofembodiment 20, as the dopant implanted at this stage, we have B, Al, Ga,In, Zn, Cd, Be, Mg, Ca, P, As, Sb, O, S, Se, Li, Na, and K.

The two substrates with the ion implanted diamond thin-films formedthereon are superimposed each other such that the ion implanted diamondthin-films face inward, and are then placed in an ultra-high-temperatureand high-pressure firing furnace. Specifically, the diamond substrates5-21A and 5-21B are superimposed such that a diamond thin-film 5-23Aformed on a diamond substrate 5-21A and a diamond thin-film 5-23B formedon a diamond substrate 5021B make contact with each other and aresandwiched between them. After that, the superimposed substrates areannealed in an ultra-high-temperature and high-pressure firing furnaceat a pressure and a temperature of no less than 3.5 GPa and no less than600° C. (FIG. 41E). In other words, the annealing is carried out underthe conditions of the pressure P (kbar) and the temperature T (K) beinga pressure of no less than 35 kbar and a temperature of no less than873K, which satisfy the relation of expression P>7.1+0.027 T (seenon-patent document 7).

After the completion of the high-temperature and high-pressureannealing, the superimposed diamond substrates 5-21A and 5-21B areseparated, thus obtaining high-quality semiconductor diamond thin-films5-24 (FIG. 41F). Because the diamond thin-films 5-23A and 5-23B seldombond with each other, the diamond thin-films 5-23A and 5-23B can beeasily separated naturally or by applying a small shock.

By way of example, a diamond (100) substrate is used as the diamondsubstrate 5-21, and electrodes are formed on the semiconductor diamondthin-film 5-24 which is produced through high-temperature andhigh-pressure annealing for one hour under the conditions of 1400° C.and 7 GPa, and then hole measurement is conducted to determine polarity,carrier concentration and mobility at room temperature. Table 19 showsthe polarity, the carrier concentration and mobility at room temperaturefor each dopant of the semiconductor diamond thin-film 5-24 in thisexample.

TABLE 19 Room-temperature Room-temperature carrier mobility DopantPolarity concentration (cm⁻³) (cm²/Vs) B P-type 1 × 10¹³ 1100 Al P-type4 × 10¹⁵ 900 Ga P-type 5 × 10¹⁶ 900 In P-type 5 × 10¹⁶ 800 Zn P-type 4 ×10¹⁶ 900 Cd P-type 6 × 10¹⁶ 950 Be P-type 5 × 10¹⁷ 1000 Mg P-type 6 ×10¹⁷ 1100 Ca P-type 1 × 10¹⁷ 800 P N-type 3 × 10¹¹ 550 As N-type 6 ×10¹⁵ 700 Sb N-type 3 × 10¹⁵ 900 O N-type 3 × 10¹³ 250 S N-type 6 × 10¹³700 Se N-type 5 × 10¹⁴ 600 Li N-type 5 × 10¹⁴ 500 Na N-type 3 × 10¹⁴ 600K N-type 2 × 10¹⁴ 600

It is seen that, as shown in Table 19, the semiconductor diamondthin-films 5-24 produced in the embodiment have a high mobility at roomtemperature. These results demonstrate that the surface of the ionimplanted diamond thin-film 5-23 is protected before thehigh-temperature and high-pressure annealing is carried out, therebypreventing the surface from being etched and providing high-qualityP-type and N-type semiconductors such as are unable to be provided byconventional producing processes.

1. A process of producing a diamond semiconductor, comprising: a firstprocess-step of implanting dopant into each of two diamonds by an ionimplantation technique; and a second process-step of superimposing thetwo ion-implanted diamonds on each other such that at least part of thesurfaces of each of the ion-implanted diamonds makes contact with eachother, and then of firing the ion implanted diamonds at a firingpressure of no less than 3.5 GPa and a firing temperature of no lessthan 600° C.
 2. The process of producing a diamond semiconductoraccording to claim 1, wherein the dopant implanted by said ionimplantation technique includes at least one of the following: B, Al,Ga, In, Zn, Cd, Be, Mg, Ca, P, As, Sb, O, S, Se, Li, Na, and K.
 3. Theprocess of producing a diamond semiconductor according to claim 1,wherein the firing pressure and the firing temperature are respectivelygreater than or equal to 32 kbar and 873 K and satisfy the followingequation:P≧7.1+0.027T, wherein P is the firing pressure measured in kbar and T isthe firing temperature measured in K.
 4. The process of producing adiamond semiconductor according to claim 1, further comprising forming aprotective layer on at least part of the surface of at least one of theion-implanted diamonds, wherein said protective layer comprises one ormore of the following: a layer of a metal comprising at least one of:titanium, tungsten, platinum, palladium and molybdenum; and a layer ofAl_(1-x)Si_(x)O_(1-y)N_(y) (0≦x≦1, 0≦y≦1).
 5. The process of producing adiamond semiconductor according to claim 1, further comprising forming aprotective layer on at least part of the surface of at least one of theion-implanted diamonds, wherein said protective layer comprises amultilayer film having two or more layers, each of the two or morelayers comprising one or more of the following: a layer of a metalcomprising at least one of: titanium, tungsten, platinum, palladium andmolybdenum; and a layer of Al_(1-x)Si_(x)O_(1-y)N_(y) (0≦x≦1, 0≦y≦1). 6.The process of producing a diamond semiconductor according to claim 1,wherein each diamond used in said first process-step is a diamondthin-film produced by a CVD technique.